The thing is, we don't know if the OmnislashV6 was able to tear Sephiroth's spirit enough to the point that the Lifestream could feasibly crush his will.
The first time his spirit was torn to shreds by Cloud at the end of FFVII, Sephiroth empowered his will right back by getting rid of excess memory and focusing on the things that really mattered to him(making Cloud his core -- no deep-seated homo issues going on there, ofc), making his will, again, strong enough to not be crushed by the current.
The creator's could honestly spin this either way they liked. Either V6 did tear him up to the point where he could be absorbed/crushed/eroded, or the fact that he was still keeping his core to him means that his will was still too strong to be crushed by the current.
So they purposefully left this open to go either way, it seems. Which is why I think that if they decided to revive Sephiroth, again, the only way to logically keep him down for real would be if Cloud killed himself, stupid as that may be.
Idk. Sometimes I just wished that they left Cloud and Sephiroth's stories alone. Unlike most, I actually liked AC/C and the Novellas. I think they closed on a good note.
If they give me something with, idk, older Denzel and Genesis, I guess I wouldn't complain as much as others.