I mean outside of the clear sarcasm this thread is showing...PS4 will be dropped at that point, I can imagine 7.X patches updating the expansions to all be uniform for the most part and then 8.0 adds more flourish and PS4 gets dropped. With how quick PS5s are shifting as well (especially with the recent sales price drop) it will be a good time to drop it off. There are other things they could do to even give PS5 more longevity whether that is FSR 2.0 but the tech has come far and they have added for instance dynamic resolution scaling.
Yoshi P would never step down or off the project especially when he commented having 2 more expansions worth of story if anything I could imagine him dropping producer role and maybe have more co-directors like he technically did for heavensward.
Other parts I think slyphs need to be a playable race said everyone and err the expansion after this should be akin to just one big animal crossing segment.
