If 'the player might forget to use the 15s GCD because they had to heal and didn't realize it's up again' is an issue, then I think they're very likely to accidentally do a 1-2-2-oops can't 3 now, because they had to heal and forgot what step they're on. And if the solution to that is 'look at your hotbars to see what's glowing' then I don't really consider that ideal, as that means taking your eyes away from the fight to look at your bars. It's a lot harder to put your entire hotbar in a place thats easy to see yet non-intrusive (due to it's size), compared to the job gauges (especially on simple mode) or your enemy's debuff list (especially if you hide ally statuses so only your stuff is visible). I don't know what AST card I have by looking at my hotbars for example, or if I got a DNC proc, or a Refulgent Arrow proc on BRD. I know because my job gauge is in a good place for my peripheral vision to notice, and even before that, I know because I can see the '+ Arrow Drawn' come up in the flyingtext that tells me what buffs or debuffs I'm being afflicted by.
A 1-2-3 on one healer is not the worst idea ever. But if they're going to do it, I expect them to take advantage of the design space (what little of it) a 1-2-3 combo offers. Branching combos with different effects for example. But since the role is 'healer', taking too much mental load away from 'healing' is not ever going to happen, and so the 1-2-3 will likely remain as simply a '1-1-1 but it's on 3 buttons'. Which would, in any other situation, be called 'button bloat'.
I'm not inherently against a 1-2-3 if it's the only possible way we can get anything for the healers. But I'd rather look at 'literally any other solution' before resorting to 1-2-3 because I find that to be the most uncreative, and least 'design space opening' of the options. The 123 is rigid in it's execution. It introduces more fail-points, such as 'you did the combo in the wrong order and broke it', 'you double-did a step and broke the combo due to needing to panic heal, and that made you forget which step you're on', or 'learning player decides to IceMage it and spams the instantcast 1, never bothering to learn how to do the combo, and refuses to take advice to change it'. Some of these are also true of other suggestions though I suppose.
I oppose 1-2-3 until the 11th hour because of this: Imagine WHM as it currently is, on a dummy. As it stands, we do one Dia per 30s, then Glare 11 times. Then we reapply Dia. If we change this from Glare 11 times to 'Glare, Banish, Luminaire' 4 times (well, that'd be 12 so skip one to apply Dia), then does that actually have any staying power as a change? Would the healers complaining about how 'monotonous' the rotation currently is, be satisfied long term with a change like this? I don't believe that to be the case. I think that we'd learn the new 1-2-3 into muscle memory before the day's out, and it'd be back to business as usual. Only, now that there's a longcast in the 1-2-3, you'd likely have more instances of people going 'sorry no swift, used it for mobility'.
There's no adaptability in a rotation like this either. With it, if you needed to Rapture between steps, presumably the combo would not break. So we'd go from 1-2-3, to 1-2-R-3. The fact that 3 follows 2 is not changed. This also would encourage something I despise about current WHM, that being, purposely shooting off a Lily for a non-healing use. I don't like that I have to overheal to prep a Misery for raidbuffs, and I sure as heck would not like 'have to fire a Lily to delay my rotation by a GCD (and get mobility), so that my next GCD during the mobility section is the instantcast Glare for more mobility'. It's bad enough that people burn Swiftcast for mobility and then can't raise someone (which is probably the reason you see so many 'Glare mages' refusing to res).
Additionally, I remember back in Stormblood, as a DRK, there were many arguments about TBN, and how it gave a Bloodspiller at the cost of a Dark Arts of MP. It worked out to be a DPS loss if that Bloodspiller pushed a DA Souleater off the fight timeline, but a gain if it pushed a Hard Slash off the fight. So you'd have parsing DRKs trying to use a specific number of TBNs, to line their timeline up so that it was favorable. This 1-2-3, would it not cause the same issue? The 'loss' back then for DRK in the worst case scenario iirc was 24dps. 24. If the 2 or 3 step of this hypothetical healer combo are even 10p stronger than the 1, eg 310, 315, 320, then delaying the rotation by a GCD by healing via Rapture would just create the same argument. And if the 'DRK is so squishy because they heard TBN is a DPS loss from Reddit' got so prevalent to become the issue it is, then 'WHM refuses to heal with Lilies, because it's a DPS loss now' is going to be even more of an annoyance I'd expect
All this, to dance around the more simple/elegant (imo) solution: simple CDs. You can gate them with CD timers, with proc requirements, or with various costs eg using Gauges. Delaying a skill for better fight alignment (eg mobility) is easier, just don't use the skill/don't spend the proc for a GCD or two. Healing doesn't cause the TBN delay thing to be as prevalent, because it instead replaces a GCD. If you're going to be going Dia, Banish, Glare as your next 3 GCDs, it does not matter what point you put the Rapture. Glare is the GCD that gets dropped from the rotation, as the lowest potency. EG: If you put Rapture first, then you have RDB, and G is removed. If Rapture is 2nd (where Banish should be), you'd go DRB, moving Banish backwards by one GCD and removing the Glare to make room for it
Again, if SE decides they want to try and solve the issue with healers, by giving us 1-2-3 combos, I'll take it. But I would expect that the forums will be ablaze with the same kind of threads far sooner with 1-2-3s, than they would with a more free-flow, 'you have more adaptability to the fight status' solution. I recommend people go to Bozja, throw on Lost Banish and another Lost something GCD (Protect, idk, just something GCD, doesn't need to damage), and try this: Glare, Protect, Banish, as your simulated 1-2-3 combo, with maintaining Dia. See how fast you can 'master' it. Then, try: Banish once per 15s (use the instance timer to time it), Dia refresh when the DOT duration timer says 18, instead of 0, and fill with Glare when nothing else is up. While the second will feel a lot more 'complex' to get the hang of at first (because we're so used to what we currently have, mostly), I feel that it's that exact complexity that gives a suggestion like that more 'staying power' as a change compared to the simple 1-2-3. That if you learn how to do the 1-2-3 without messing it up super fast, then it's going to become rote memorization much sooner than the other potential solutions.
Damn this got long huh