I think this is due to them looking for a loophole in the lore they’ve created. Chronologically bahamut, phoenix and hydaelyn are the three primal you first meet - 7th umbrella calamity for the first two and hydaelyn tempering the player in that cutscene. It doesn’t have to be a bahamut, but as all the demis are obnoxious flappy things so may as well use it as a base for demi hydaelyn’s paper thin disguise.

Since the majority of job adjustments/additions have been specifically in burst, and there are few non-healer capstone abilities that aren’t burst damage, it’s basically cemented in there. I think there’s also the cost/work balance to consider. Animations and attacks for each added summon (forced to be added in 3s by the endwalker changes) are far more work than say changing bootshine’s animation and adding 50 potency. Not to mention that the more summons added the less each will be seen under the current paradigm. At some point they’ll just say it’s not worth it and it could well be that point is 3 normal summons.