If there is indeed a next Xbox, I expect it to come sooner than PS6, as Xbox Series is pretty much a lost cause and dead in the water at this point.
If I had to guess, I'd say Nextbox in 2026, PS6 in 2028. Sony will want to prolong the generation to recoup their ever longer and more expensive software development cycles, and they'll also want to wait for the hardware needed to noticeably one-up the PS5 Pro to become affordable.