Eh, given the lukewarm reception of Dawntrail and Endwalker, I feel it's less about running out of spaces to go and more about losing player interest and therefore revenue to develop future expansions.

Realistically speaking, I don't see FFXIV going beyond 10.0 without major changes

Too many jobs, too much bloat, old content basically becoming dead once the expansion is over outside of niche play.

Not to mention, getting a new player excited to play endgame with you is becoming more and more difficult, ESPECIALLY for players who are really only interested in endgame/fight/raids.

Sure, there are story-skips..but by 10.0 it'll be "Grind through the story your first month or two or three, or pay more money in this pay-per-month game so you can play with friends" is not going to be that exciting considering alternatives games.
Basically, for players who don't care about the actual story, the problem will come when it's factually cheaper to buy a story skip than pay the month(s) of sub fee needed to do the story.

For casual players, 2-3 months is a pretty big ask just to get to a level to play with friends in current content.