I don't agree that the devs here would try to make it a mandatory part of PvE progress. They're not the WoW devs.
Here side content pretty much remains side content, optional to do depending on what you want to accomplish. The only reason Crystal Tower had to be moved to mandatory for MSQ was because of what happens at the very end of 5.3. Up until that event, players who hadn't gone into CT wouldn't have a problem enjoying the Shadowbringers story.
I still don't understand why Return to Ivalice was required for Save the Queen, though. I haven't completed Save the Queen yet but the Ivalice story isn't relevant to understanding what I've seen so far. From what little I've read on how Save the Queen concludes, it still doesn't seem to be relevant to understand the story anyway other than a certain character from Ivalice reappearing toward then end? That's not a problem, new characters get introduced all the time. They could have put in a "I remember you" easter egg dialog line for players who did Ivalice and left it at that.