That's extremely generous. If the current decline increases steadily, the ps4's post ps5 sales will be very close to ps3's post ps4.

With PS5/XB2 launching at 500/600$.... why wouldnt a cheap ~199-249$ PS4 keep selling?

Arguably its game lineup in 2020, is better than the Switch's this year.
Also alot of multiplat games, will be cross gen, early on.

Theres still plenty of reasons to buy a PS4, if your without one.
Its the game consol currently with the strongest game library on the market, and it still has alot of bangers comeing.

Because new consoles won't increase the ps4's sales and it's decline won't stop even with pricecuts.

Extremely subjective follow ups that bear little relevance to sales, since new sofwate barely affects hardware at this point of a console's life.

To be fair we don't know a lot about the Switch line up this year yet. Until they show us a bloody direct! X(

Never and if it does I join the group of Nintendo haters and call it a handheld so it doesn't count =p.

But my serious answer is that it has the potential.

In January 2017 a lot of people dissapionted with the price of the Switch and you got the typical 'why will people buy a switch when you can get a PS4 for a cheaper price?'.

3 years later and basically it is doing better than 99% predicted and still around that price that is a huge achievement. 

Also it's software is not just selling well it is dominating,  Smash/Breath of the Wild/Pokemon SS/Ring fit adventure/Mario Kart are doing better than expected and some are outselling the best selling PS4 first party software while almost still being full price and on a smaller userbase.

We are now almost reaching the longest time between two directs ever (176 dayish) a bunch of surprises first party and third party can get announced for the next coming years that potentially can keep the switch selling for years + pricecuts.

The only thing that is dissapointing are the switch lite sales ,people seem to not want the handheld experience.

I wouldn't count on it. PS4 is dropping hard and will be behind last year by over one million by the end of March already. At this rate the PS4 will sell about the same as PS3 did in 2012 and will probably also sell in 2021 about as well as the PS3 did back then in 2013.

With practically nothing scheduled beyond March 20th, I am starting to worry that Nintendo have little to show for themselves this year, in which case 2020 could be down significantly YOY and mark the start of a steep decline for the Switch like what happened when Wii stopped getting games in 2011.

I certainly hope that's not the case, but their current lineup (or rather their almost total lack of one) doesn't exactly inspire confidence.

This year will give us the answer if the Switch can match or come close to the PS4. For now, it is slightly up compared to it in the same timeframe, and it has to do over 20.3 millions this year to beat the PS4 again this year. We are not so sure about the numbers for 2019, VGChartz has it to 19,3/19,4 i think, which is a pretty great performance (sad it didn't do over 20 millions tho).

Reasons this year could be up is :
-Momentum being up from last year already with no game
-Presence of a huge system seller early in the year, targeting an audience that the Switch hasn't before
-Nintendo starting to experience more with the bundles and exclusive collector design as seeing with the AC Switch
-A new switch model. I know they said it won't happen but they always say that the year it happens so...
-Presence of the lite for a whole year
-Potentially a price cut
-Full year in China

BUT, i've seen some analyst on Twitter saying the Switch would be down single digit this year in the US, that said, they don't know the whole plan for the year and it is currently up so it is still pretty much based on nothing and even if it is, could be saved by Europe and Japan. There is still some factors that could let it be flat over last year :

-No price cuts, again
-Coronavirus not slowing down and affecting heavily the production
-There is not much games coming this year (which i am doubting tbh)
-No new revision

With that said, even if there isn't a huge game like Pokémon at the end of the year, it will have a much better momentum for the rest of the year to compensate and Nintendo could still pull it off at the end of the year with aggressive promotion. And on top of that, Switch not reaching 20 million for example for lack of price cut, could only mean it will probably have a long as heck life cycle and drop really slowly.

So in the end, there is still so much possibilities, Nintendo can do so much more with the Switch, it will depend how much they want to push it.

I am seeing the PS4 ending at around 122 million copies, selling around 9 millions this year and 5 millions after that. PS4 is on a faster decline than the PS3 and the PS5 will be retrocompatible so that will factor a lot. 

For the Switch, i think it will come short of it, right now i am seeing it surpassing the Wii by a comfortable margin but falling short to reach the PS4, let's say around 115 millions. It will really depends how quickly Nintendo pulls the plug on this one. They tend to do so too quickly sometimes, like with the DS with the arrival of the PSP or the 3DS for the Vita. This time there is no such things cause the Switch is on a category of it's own, if there is anything, Sony and MS are battling for who is going to attract the switch audience to by their console as a secondary option (MS porting games and rumours of game pass, Sony tempted by remote play). And also, Switch won't die as quickly as the Wii did looking at the healthy momentum and no sight of price cuts, while Wii was getting already negative opitions by gamers by then.

Let's just say it's nice to see 2 extremely successful consoles in the same or almost same generation (poor ms).

The bigger reason why SW will never beat PS4 is looking at PS4's tail.

FY
16/17 - 20 mil
17/18 - 19 mil
18/19 - 17.8 mil
19/20 - 13.5 mil

Don't see SW holding nearly as well

To be honest, I'm starting to doubt Switch will sell as well as it could have and pass the Wii.

It's less than a month til Animal Crossing and there don't have a single major game with a release date beyond that. It really feels like they've run out of software and are hanging the system out to dry, like they did with Wii in 2011 and Wii U in 2016.