The PS4 is almost dead and Sony will ensure it's completely dead by 2022. The amount of PS4's that will hit the second hand market once the PS5 is on retail stores will make it impossible to reach the 150 million mark. The PS2 had extremely long legs but the current situation is completely different because the PS3 had a rough start and nowadays we are way more used to planned obsolescense.
But the PS4 had an amazing run. The fact it has surpassed the Wii and the OG Playstation and is approaching Gameboy numbers is simply amazing for our times, when everything goes so fast.
No, they won't kill PS4 that soon, that would be stupid. Yes, PS2 had more legs but also the other PlayStations, that lasted for 10+ years. 10 years would mean late 2013. And PS4 isn't dead at all, they still are breaking records in the current quarter, look at the resetera link included in the IP.
After PS5 release PS4 won't only receive now the typical annual games like FIFA or Just Dance, thanks to its huge install base and the PS5 BC, PS4 will also receive a good amount of -crossgen or not- games both indies, big 3rd parties and even some ones published by Sony. Many of the games we saw in the PS5 conference were crossgen.
On top of that, PS Plus and PS Now from PS4 users generate a ton of money for Sony, so they may want to keep that for the ones who don't upgrade to PS5 during several years, for the longer time possible, something they can do even without releasing new Sony games.
In addition to this, aligning sales PS4 follows closely the PS2 even if at this lifetime point PS2 was cheaper. Sony still has PS4 price cut cards under the sleeve for when they needed to boost PS4 sales and reach new markets when crossing the permanent $199 magic price point.
I think they may not reach 150M, but I think there's many millions of PS4 to be sold and that they will end closer to PS2 numbers than to Game Boy+GameBoy Color (118M, they will pass it this year) total sales numbers.