Two questions:
1) But why?
Why does BLU have to be more than what it is designed to be (side content)?
The reasons you give for why they would entertain the idea are generally vague ("make the game better", "save resources"), self-serving (content you personally want over things you don't like) or appeals to nostalgia and emotions ("traditional job", FF history, "it's my favorite from the series!"). Going from your thesis it sounds like you have a very emotional attachment to the job concept or more of an obsessive hatred for the limited job idea. And while I sympathize with the nostalgia/emotional angle to an extent, none of these give me a good reason why SE has to further invest in something they clearly don't want or need.

2) Why change BLU when simply adding a standard caster job would accomplish the same thing?
Let's ignore the reality that they plan and allocate resources to content/expansions years in advance and have at the very least weapon ideas for future jobs (potentially caster) that could fill any gap. Why not simply add a caster for the 4th spot at a future date and keep BLU as they intended? Why can't Geomancer or Green Mage fill the same niche as a neutered BLU would?

I think I've asked this before, but can't remember getting an answer that didn't involve some sort of special pleading or downplaying the difficulty of balancing jobs and content.