I mean, is “no return” even on the table, where “Sasaski,” Seto, and Past Nanasawa just continue in the timeline they’re presently in?
Why not?
Theoretically it can happen, of course, but there are reasons why that’s probably the least-common time-travel variant (the traveler staying in the future may be slightly more common).
In this case, as has been suggested, that eliminates present-day Seto as a character.
It fails to resolve the “one person/two identities” conundrum.
It fails to exploit the traditional climactic reveal of time-travel stories that shows how the narrative present has been changed by the traveler’s experiences.
(Or perhaps only the traveler has been changed—it could be like Dickens’s A Christmas Carol, only instead of saying, “Merry Christmas, one and all!”, MC comes back and says, “Seto-senpai, I want to fuck you!”)
Your reasoning makes sense and I personally do want to see adult Seto again. But also, this manga is handwavy enough with its time travel setup that I can totally see it just going the "Fuck it, "Sasaki" is happy here, so why return?" route. And if that gives me more polyam triad shenanigans, i'm all for it :D