I have a pretty big PS4 library, so my PS5 is not only to play PS5 games but PS4 games as well. I will delay my PS6 purchase as much as possible. There is no Sony exclusives I can think it's an instant buy for me, I only want to play their single player games and I can honestly wait for a few years to play them
But having ability to play old library is one reason to purchase. I think PS6 will be able to run your PS4 library too, even if it's just emulation. If this is the case I can replace my PS5 for a PS6 granted they don't sell it for any crazy prices