I'm really hoping that analog isn't relevant to your wider point, since, if anything, the other casters would be more interesting if they didn't spend so much of their reiterations apologizing themselves right out of role identity.
And, if WHM were putting out a form of throughput that wasn't so quickly capped / made redundant... that'd honestly be fine. The core issue is literally just that (A) there's no non-burst and non-scripted healing to be done anyways and (B) despite its "big/pure heals" identity... it's typically had weaker or merely competitive heals in practice.
That said, on all other counts...
...Agreed.WHM has several design....directions? "Identities"? ... Whatever you want to call them, they s u c k.
And yet... BLM really does still do fine. Top rDPS for every fight. Top aDPS on many. Yes, modern fights are more demanding of mobility than before, but... cast times are no more a fundamental issue than positionals or any other form of contextual complexity.The reason the caster DPS have so many instant casts is because they would be unplayable without them.
Like... I agree with you that WHM deserves a rehaul, but... I don't think the 'issue' you're focused on is nearly as great as you make out here. It's instead a simple matter of parity, where other jobs simply have more.
And even if, say, the devs ended up fixed on the idea of WHM being a high cure-ppgcd healer... as long as they made up for that with offensive potency and compensatory features so that any healer pair with them had roughly the same offensive ppm in practice (WHM probably "chadding" over AST or SCH, but w/e)... that'd be fine.
For me, my main issue is that WHM's only identity is Generic Healer™ and that it so often just hasn't been balanced against the other jobs. But so long as fights are consistently demanding of mobility, that doesn't mean that you need to give every job the same amount of mobility, since you can just compensate for that lack of mobility in other ways. Similarly, even if more of a given healer's curative ppm would be GCD-based, as long as its total curative ppm and offensive ppm would be balanced in practice... it truly does not matter; you can quite simply raise potency in compensation, and the only "problem" would be that the "GCD healer" would be more rewarded than most for doing zero healing over raid buffs and for healing when there's nothing to attack.