I doubt it'll come out that soon, and agree more with the 2020 assessment considering things like cost and performance increases available to them the longer they wait.

But just imagine if this 2019 prophecy is right, and Sony find a way to make Xbox pull a Sega Saturn. That would be horrible for everyone.

Well, let's look at PS4 and PS4Pro GPUs

PS4 features a 1152:72:32 part, so it has 1152 shaders and 72 texture units, and it's based on Pitcairn, maybe Tahiti architecture.

PS4Pro has a 2304:144:32 part, with 2304 shaders and 144 texture units based on the improved Polaris architecture with some Vega enhancements.

What we see is that PS4Pro's GPU has more than twice the power of a PS4, yet what is the best it can do? Increase the framerate (which is also CPU dependant), increase the resolution from sub 1080p to Full HD and add slightly better lightning and textures (enhancements that require Digital Foundry's close up analysis to notice, because we can barely see them during gameplay) or run the game at higher res and then make a 4K checkerboard interpolation. That's not much for more than twice the power, don't you think?

Now let's look at Vega 64, AMD's counterpart to the Nvidia GTX 1080, and we see that it's a 4096:256:64 part featuring 4096 shaders and 256 texture units. So, even accounting for its architecfural enhancements, Vega 64 is not twice as powerful as a PS4Pro or RX 480/580 GPU.

Sure, it will be able to run Vanilla PS4 games at native 4K which, given how much do both Sony and MSoft talk about it, seems to be the goal for next gen consoles as far as resolution goes, but can it do that while also displaying better, more detailed graphics to a degree where we can see them without resorting to DF or other kind of exhaustive comparison sites?

I find it doubtful.

Side note: I've taken the GPU info from both this wikipedia page with all ATI/AMD GPUs and also TechPowerUp's database, in case you want to check them out.

This is what I wrote:

IF Sony can offer 4K at 60fps on every single forward conpatible game and the majority of exclusive PS5 games

I wouldn't expect every single true next gen game to hit 4K/60fps, but I would expect every current gen game to hit 4K/60fps. It depends on what devs prioritise next gen. By simply removing more advanced techniques they'll be able to hit the 4K/60fps but if they choose more advanced graphical enhancements over 'only' resolution, I'd say 4K/30fps because resolution is STILL the biggest selling point. The reason I've said 'the majority' is because I believe Sony will opt to try and hit both bulletpoints in order to make sure even a more powerful console won't have those bullet points to use as an indication it's better to Joe Blogs. 

Just check any official Slide from AMD and compare

Surely when it arrives it will be in November of whichever year either 2019 or 2020.

You're comparing directly by theoretical numbers, and even then don't factor in important ones like the clock speed, you should see that I used FLOPS to compare different GPUs (of course there are architectural differences, TMUs and ROPs that affect the overall performance too) and it's better than simply using shader cores and the like. I mentioned a 12TFLOPS GPU for PS5 in 2020 in my original post, which is exactly twice the power of Xbox One X, even without accounting for its architectural enhancements.

Xbox One X, from what I'm seeing, is running half of its games using a CBR method (or some reconstruction technique, or even dynamic resolution) to achieve 4K and half at native 4K, most first party MS games are native 4K, however. It might change when the console gets released and more games support it, though. Barring the resolution upgrades, the One X also has other visual improvements to the image quality, like higher resolution textures and in specific cases it utilises a higher quality of visual settings e.g. better AO solution, higher quality of depth of field, better LODs and better draw distances, just to name a few. If we consider the latter, half of PS5 games should have no trouble running at 4K while leaving a signifcant headroom for improving visual fidelity. As for demanding games, I absolutely expect some games to not make native 4K. I don't remember there being a single gen (after Genesis/SNES) where every single game was running native resolution. Developers will surely be sacrificing image quality if need be, it's been done before.

I think comparing PS4 Pro enchancements and using that as a basis for next-gen console visual upgrades is a bit deceptive because games will be developed from the ground up for next gen consoles and it's obvious that these mid-gen refreshes aren't being fully utilised, probably never will be because of this. We should see significant changes in lighting models or probably entirely different rendering techniques, though I may be reaching here. One thing I did not consider while contemplating this, is that consoles already use a portion of their resources to for GPGPU tasks to make up for the weak CPUs. As the CPU upgrade will be a noticeable jump from this gen, we should definitely see less of that and more free resources for developers to use for graphics.

Well, we both agree that there's no way Sony can launch PS5 early 2019 because it wouldn't offer a big enough jump in performance.

Beyond that, you believe that a 12 TFlops GPU, so something like AMD's current Vega 64, would be enough of a jump while I think that it wouldn't be enough. Who will be right? Only time will tell.

There's still a problem, tho. Will AMD be able to produce a GPU that powerful but with a low enough TDP to make it viable for a console? I mean, it's impossible to do that on 12nm, so they'll have to wait for 7nm. Will it arrive in time?

I want to co-sign this post. With the jumps on visuals getting shorter  and shorter the time between gens should be getting longer. Performance matters more than graphical fidelity at this point. And to be honest this gen has just started to get going. I imagine we wont see PS5 until 2022 at the earliest.

Indeed. For console makers, as for other consumer electronics producers, unlike for gaming PC market, where they are optional and/or niche, there are two factors that are as important as raw performances, performances per watt and performances per dollar, those are the factors that give the green light after the first parameter, desired performances, has been reached by mass-produced components.
And as I partially wrote in another answer to gcwy, there are two factors, demand and customised APUs, less affected by market turmoils, that could favour an acceleration by AMD and reaching the right price, but other factors are against it (mainly all the legal obstacles made by Intel that AMD had to overcome in tribunals and deals to be able to increase CPU production and that are still affecting it for the accumulated past delays and lost potential market share increase during those six years when AMD desktop CPUs always outperformed Intel ones in each market segment including high-end) and crazy and volatile GPU prices and demand due to cryptomining, while in theory in favour of tech acceleration, are OTOH against reaching the right price in the short term both for GPUs and RAM.

I see a lot of talk about XBox, but there are two systems that seem absent from this discussion: Switch and PC.

The XBox isn't that high of a threat for Sony. Mircosoft will either shift to PC to protect Windows or falter so much that it wont matter what they do (which seems to be the case as they have been canceling so many games). Switch and PC put Sony between a rock and a hard place. Switch offer mobility allowing you to take console games on the go. Wolfenstein 2, despite being released a few months later, is a game that is on both systems, but the Switch allows you to take the game on the go. On the other hand, the PC is more powerful and is about the same price. You can build a system comparable to the PS4 Pro without spending too much and you have access to a larger library of games.

Right now, Sony is losing its niche. It uses to be that Nintendo dominated the low end of the market, Sony and Microsoft dominated the middle and PC dominated the high end. Now, Switch and PC are absorbing the middle. Sony (and Microsoft) are overly reliant on third-party titles to move systems, but more companies will port to these two systems in order to recoup their enormous budget. Sony has shown they can't compete with first-party titles, so they are in trouble if they can't get that area up to Nintendo levels soon.

A lot of commentors said, "why release soon when PS4 is doing well?" The issue is the market is very future focused. PS4 is doing well now, but there will be a time when Sony has to move on. Console generations are a 5-7 year endeavor, so if Sony doesn't get it right, they will have an awful few years. With gaming being the major sector and the company having a lot of debt and poor liquidity, the company could be a going concern if they don't get this right.

If Sony waits until 2020 to release a new system, this means Nintendo dominates the market and PC will being to absorb Sony's customers (of the two, PC is the bigger threat as there isn't enough to differentiate the two). Why invest in a PS5 when you built a PC in 2018, 2019 or 2020 and all the games are coming to PC anyway. More developers will be looking at Switch and PC first because these areas already have the install base. Publishers have been looking more and more at PC as the user base has grown and game budgets have expanded. Additionally, you have more than a few developers lamenting consoles as they feel they hold games back. In Japan, developers will quickly move to Switch much like they did with the DS and 3DS.

So where does Sony release? They release too soon, they will piss off core fans as a lot of PS4 titles will be moved to PS5. They can't release late as Switch and PC will absorb Sony's market and steal development resources. The best time to release is early 2019. It only gives Switch 2 free years on the market and may stave off the growing PC threat. That said, Sony needs to start looking at a way to differentiate the Playstation line. Sony's niche wont last much longer.