The story is the fundamental underlying feature that holds the whole game together. It's what distinguishes one game from another. And it's the biggest draw that gets people to choose to play a game, as without it, there really wouldn't be anything worth doing. Without story, we'd be just randomly bashing one meaningless nameless mob after another for no reason except that it's there.

I absolutely agree that it's vital for all players to go through the entire story line. And that the expansion's story should continue on from where the existing story leaves off, meaning the existing story needs to be a prerequisite for it. I don't, however, see any reason for it being a prerequisite for unlocking the expansion's jobs.

It seems as though the job quests being located in Ishgard is the only reason they're locked that way, but it doesn't explain why SE is choosing to put them there. There have been plenty of indicators that Machinist was going to be in Limsa Lominsa, and the most natural place to put Astrologian is at that observatory in Coerthas. I'm not as sure where Dark Knight would go, but SE could come up with something. It seems more like they're being pushed into Ishgard because SE wants to gate them behind the story rather than that they're gated behind the story because they're there.

This isn't a matter of the story saying the jobs are in Ishgard so we have to get there first before we can learn them. It's a matter of SE deciding we should get there first, so that's where they're being placed.

(Nor is it the first instance of the story seeming a bit contrived to fit where SE wanted to place something. The Rogue storyline would have fit far better in Ul'dah than it does in Limsa, but it's in Limsa presumably just because initially Ul'dah had the guilds for 3 battle classes and Limsa had 2.)