I've found there's some great variety in healers, tbh. Some are solid enough, some good in their way, some great but a bit sloppy, and then there are those who probably watched some popular youtuber explaining "this is how you should heal" and let me die without healing me ONCE, not bloody once, when I've been tanking the boss for the past 30 seconds, steadily taking damage. And it happened again and again.
You can't please everyone, that's just facts, not even your own crowd when making games, especially when the crowd's big as in this game.
But I think it's simple: "you are healer" + "you lack challenge due to whatever (in this case tank not dying easily enough)" = "do harder content".
I've healed and tanked as Blue Mage my bit share, and that's a challenge even in normal mode encounters due to how disorienting it is, because there's not proper incentive to do that regularly enough, unlike with the regular job content (roulettes). If we had something like a BLU-only roulette, that might go long way for satisfying this hole for certain players.
Oh and notice the timing of this topic: early Endwalker. Of course, when there's little content that's both relevant AND challenging enough, just the ex trials and that's it.