I think that the PS5 will launch in 2020 or later. The main indicator for me is Rockstar. Since we are getting 2018 RDR2, we wont get GTA6 in 2018 too.
Rockstar want to make as much money as possible with GTA6, similar like GTA5. So they will launch it one year before the release of the PS5 on the current consoles. When the PS5 arrives they will publish a remastered version for that console so that they can double dip on many hardcore gamers.

They would be stupid to release it that early.

I second this. I can see GTA VI dropping on current gen consoles in 2019 and 2020 on next gen consoles (launch).

PS5 could be fully compatible with PS4 and PS4 Pro plus have a greater integration of VR (no second box) and be sold at a premium price at first. It will take nothing from PS4 sales for those with 1080p tv's who aren't interested in VR until its software base has expanded of native PS5 titles. Likely based on Ryzen CPU architecture or better.

Xbox One X has failed to take off significantly and I feel a launch is most likely autumn 2019 or 2020 because Sony aren't under as much pressure. I think there is more pressure due to Switch which has shown the hybrid console format is very popular with a huge number of people.

If they do something like a combined Nintendo Switch competitor that also doubles as the VR visor for PS5 they make launch both at the same time so 2020 seems more likely.

I forsee the standard PS4 becoming just a budget console in some markets especially South America etc but in more major markets like Europe, Japan and US the pro model could be entry level. Otherwise the line up will get confusing if there is ps4, ps4 pro, ps5 and a Switch competitor (PS4P or whatever it will be called).

Resting on your laurels is never a good thing and that's when the market can be taken away from you. Switch has become a significant competitor in a relatively short amount of time. I just don't see the problem with ps4 because if its x86 based all your existing software is likely compatible anyway its not comparable to the situation where the ps2 had to feature the ps1 chipset or early ps3's had ps2 chipsets which added to manufacturing costs. They can pretty much offer ps4 compatibility without cost. Maybe its sufficiently powerful that it could even emulate the ps3 at full speed although I doubt that would happen as Sony would prefer you to buy new games rather than play your old ps3 games in addition to your existing library of ps4 games.

Why would it launch this soon? The PS4 has this gen locked up alreay, why release a new console so soon after the PS4Pro, when they can just sit back and enjoy the sales's inertia? The XBOne X didn't hurt PS4 sales, and neither did the Switch, it has nothing to worry about.

A PS5 doesn't hurt sales either, so why not?

The PS5 would cut off the sales of the PS4. It would still have legs, no doubt, but why bother when you can just wait until the sales slow down by themselves, then release the PS5? A PS5 launch too early would mean a lot of potential sales lost.

Mathematically it works out.

If PS4Pro was a half generation 3 years after PS4 launch, then 2019 would see PS5 but I think it's too soon. Although the it would give them a year on MS.

Because you're not an asshole bent on holding back progress at all costs? Sometimes I like to hope good things.

Why would it lose them sales? I don't get that part. It would lose them sales on PS4 Pro and PS4 but those will go directly to the PS5, in addition to all the double dippers.

2019? WHATT!