Worth noting that the PS5 is, for the most part, backwards compatible with the PS4. This was not the case for the PS4 when launched, which was not compatible with PS3 games.
If you wanted to play PS3 games after the PS4 launched, you still needed to buy a proper PS3. Meanwhile, if you are the kind of person who might be interested on playing PS4 games, why would you buy a 300$ PS4, when you can instead buy a 500$ PS5 that will play most PS4 games and last the entire generation? (or 400$ for the digital edition).
Therefore I predict that the PS4 will have very small legs; not many people will buy a PS4; unless it gets a massive price drop (dropping to like 200$ or something dirt cheap like that)
Then again the PS2 did have some great legs while the PS3 was backwards compatible early on, but that is probably because the PS3 was outrageously expensive early on, so even if it was backwards compatible you were better off buying a super cheap PS2 slim and then later waiting for a PS3 price drop.