Quote Originally Posted by Cassar View Post
(Of course, this would imply that the Endwalker raid would have to be mandatory for the MSQ but I think they could very well do that)
Time travel is a free pass for continuity. Everything we have done or will do in Elpis/Pandæmonium has already objectively happened, whether we've experienced it yet or not.

That said, it's unlikely that they would tie it together quite so strongly, but they wouldn't have to make the side story mandatory to enable it if they did.