Sony will need to figure out a way to smother the PS4 in its sleep if they want to push a PS6...And I'm pretty sure they've already tried, to no avail.
The PS5 has truly been a victim of the PS4's success; unless the PS6 was some kind of revolutionary gameplay experience that you could only get through its hardware (i.e., something related to the interface between player and game, not a performance upgrade), marginal gains in framerate or poly count is not going to sell new systems anymore. The big leaps in graphical fidelity between system generations appears to be over.
And even if a PS6 could manage a similar jump and have photo realistic worlds to scale with our own with millions of characters, would consumers even want that? Nintendo's successes with hardware in the last few generations have proved that innovation in ways to have fun is a major key to selling games and the consoles you need to play them with. (Of course, I guess having a good library and deals with established game franchises, as well as a software and licensing environment friendly to developers, can help, too.)
I have a feeling Sony might want to see what Xbox has up its sleeve for its next generation console, too, and what success/failure that console might see...