And creating more of BLU only centred stuff is only going to add to the extra developmental resources that's going to far exceed what just being a regular job will be. I'm not asking for all status effects to only be usable with BLU on every boss to be vulnerable to it (pretty much every other job already has access to it on oGCD anyway) but then that's a revision they have to make, but knowing how long FFXIV has been going on for I doubt they'll overhaul the Battle system any time soon, making those combos practically useless in any fight.
You're asking why they can't make the job compelling yet completely missing its flawed design. It's trying to work as a solo job, in a MMO where you party up for content, to do solo things, with a kit that's a mish-mash of 100-130 potency spells, a lot of which are copies of each other, status effects that have select applications in each fight (just like every other job). What part of the kit is compelling? Missile? White Wind? Doom? Most of its truly broken kit is unusable when it matters, even in Masked Carnival and White Wind is a niche Heal AoE, with a severe mana drain.
You want 100+ spells, yet most of them are gonna be carbon-copy spells with potency and an elemental property attached to it, maybe a different way to apply a status effects and maybe a few broken spells here and there that bosses will be immune too. That's quantity over quality and you know it. That is not compelling, that's just an overabundance of copy-paste that's more for flavour than actual depth.
As I said before, you can have the current planned BLU kit while still being able to have the ability to be unlimited. The very implementation of BLU is a paradox, it has no place anywhere. It is literally a street/circus performer. There is no other way to play BLU because the very way it is designed has no feasible or practical way of playing it.
You say it "takes dev resources" yet so does everything in this game. Creating a new race for example takes way more effort than a job and resources for far less of a gameplay impact
You're contradicting yourself with what you want and the current application of BLU and that the Limited job system does not work, with the claim SE should have it stay limited out of laziness.
Btw, Dark Knight is not a DPS, has no HP spend or degrade over time, and uses Mana and is a tank in FFXIV. Is that keeping to what a DRK is meant to be? Summoner only has 3 egis to choose from (realistically 2) and gets to summon out only Bahamut, it's mostly a DoT mage with tacked on pet mechanics and a semi-summon function, yet does not summon any other classical summons, has no big diverse summon attacks and constantly being locked to 3 egis for the past 5 years. Is that keeping true to the SMN identity?