What other MMO has done this, and why does SE think it's a good idea?

[Snugglebutt]

I dont agree with OP, but his point is sky and sea were not needed to unlock the jobs of the expansion like being able to go to ishgard will likely be.

The restriction of the jobs in the expansion are not locked away behind the main story per se or having access to the locations in the expansion, they are locked behind the prerequisite of owning the expansion and having a character leveled to 30+, they all start in preexisting cities aka Gridania, Ul'dah and Limsa I think. Access to the locations within the expansion are locked behind the story though just like all dungeons, trials and raids are locked behind quests and most of those are locked behind main story quests...similar also to some nations of Limsa, Ul'dah or Gridania are locked behind main story airship travel quest, chocobo riding and becoming part of a GC and more are also story unlocked.