well but with 99% of PS4 games working on PS5 and PS+ being a thing it could be worth it to provide an ultra cheap option that's in impulse buy territory to get people locked into their ecosystem. Even if it only results in PS5 sales years later it's still worth it for them. Those cheap buyers are then more likely to get a PS5 when it is cheaper.
People buy the PS4, subscribe to Plus and are likely going to buy a PS5 later on, specially with the PS Plus Collection, they just move when they can, sell their PS4 and still have 18 of the best games on it available on their PS5 as soon as they get it.
Sony should really be making the PS4 as cheap as possible to secure the most people on their ecosystem to move for the PS5 down the road.