Which is why the solution should not be 'leave one exactly as it is', but 'balance the extra complexity for one of the healers in such a way that it is mostly ignoreable for those who don't want to interact with it'. Look at it this way, if we have only Dia and Glare for the majority of our GCDs, then any movement that costs us a Glare is punished even harder, because that's the majority of our damage: Glare. Dia doesn't need to be 2.3ish Glare's worth of damage, it being that high just makes missing ticks of it (due to being bad at DOT management for example) more punishing. It could be 450p total instead of 715 with the missing potency moved elsewhere in the kit. But because SE has removed so much of our kit now, all of the potency has to be forced into the few buttons we have left.
Besides that, the 'leave one exactly as it is' has been argued to death already. For my part, my reasoning for why it's not a great idea is twofold: one, the job that usually gets thrown out there as 'the one to leave alone', WHM, previously did have more buttons than currently. 'Keep it as it is, for those who enjoy the current gameplay' didn't seem to matter going into SHB, so I equally don't see it as a reason that applies now, at least, no more than it should have back then. Secondly, 'keeping one simple for those who like simpler healer design' does not have to directly translate to 'keep it exactly as it is, with zero changes at all'. We already know that SE will add stuff for the expansion, so that alone will cause 'changes to the job', so that falls flat IMO. I would agree that there should be a spectrum of 'complexities', going WHM>SGE>>>SCH>>AST probably, but that spectrum does not necessitate WHM stays exactly where it is. I made a chart a while back, here is a retrofitted version to illustrate what I mean:
This is something along the lines of what I'd like to see for each job's complexity. Green is 'complexity of getting your healing out', red is 'complexity of dealing damage, either by direct skills or buffing allies', and gold is 'the absolute theoretical maximum complexity, which you'd run into if you were aiming to get rank 1 parses'. The reason for the SGE 'gold bar' being so long is because I would like to see it's 'damage>healing' identity properly realized, so optimizing that would theoretically be quite technical. Bearing in mind that the DPS classes, not shown on this chart, would have their bars fall anywhere between the 4th and final lines (with BLM probably having it's 'gold bar' going off the side of the chart), I'd say the proposed complexity totals for healers, even AST, is quite respectful of casuals and of the players that the 'one needs to stay the same' intends to benefit.
As the chart shows, I would like to have WHM's healing AND it's damage both be simple and easy to execute, as it is the case now. But I would also like for there to be a bit of optional depth, wherein optimizing your healing and damage adds up to a complexity total larger than the sum of it's parts. This would not affect it's simple 'easy to get into' nature for casuals, just be an extra minigame for the hardcore optimizers to mess with. The minigame of 'use Lilies to prep Misery for raidbuffs, but still make sure to use them in good places for healing' is a good example of one of these optional optimizations
As Skellington put, and I have mentioned elsewhere: I am a good example of why this idea (keep one as is) would not work well. I want to have more complexity in my healing downtime, to keep me more engaged in lower difficulty content. Doesn't matter how good I get at the game, EX roulette is still the fastest way to cap my tomes. So, I have an alt I use to reclear Savage fights with a friend, so she can 'skip' her static to the later fights they want to do (because they no longer need gear from earlier ones). This alt has one class levelled. For the sake of the example, let's say it's WHM. I do not believe that watching the other 3 healers, that my alt does not have levelled, getting more complexity to their downtime damage rotations while WHM stays exactly where it is, is going to feel good. Doubly so, when people suggest that 'if you want more complexity, play one of the other healers'. It takes time to level another job to max, I don't believe that telling someone they have to grind another job to max is a 'solution'. It's like if DRK was dying too much in dungeons because Dark Mind only affects Magic damage, and the suggested solution is 'level another tank'. Maybe I like WHM's aesthetics, or it's healing-side gameplay, or my personal reason, 'I really do not want to have to do the AST opener over and over while progging because it makes my hands hurt', but whatever my reason, I'm apparently going to be told 'nope, it's gotta stay as is, play another job'. Compared to just making all four healers have extra complexity, and said extra complexity be optional, I just don't see how it's a valid solution.
Besides, if someone's going to be a melt and complain that 'healer didn't do their optimal rotation in Aetherfont', report them. But also, if they're going to get whiny because a hypothetical reworked SCH that has 3 DOTs of staggered duration (30/24/15 for example) forgot to refresh 1 or 2 of their their DOTs for a couple of ticks (movement for example), then I'd expect they'd also be whiny about the SCH in the current design missing their one DOT for a few ticks. Doubly so in fact, because 'it's easier to refresh one DOT than three' or something. Point is, if someone's gonna be a dick, it's not going to be made less likely or more likely to occur based on job design. The healers could have passive 'you deal 100p damage to any enemy you're within 10 yards of' and they'd be complaining if you ran out of range to do a mechanic, y'know?