Yes, but if you go back in time a bit, there was one example of this being the case: HW MCH. And to make it function correctly, rather than have it as an actual combo, it functioned more like what we know now as 'Clean Shot Ready'. Getting to the 2 or 3 in the combo removed the cast time, yes, but the first step always had the cast time before chaining into the second, unless you used Rapid Fire. Still, point stands, a cast time action that functioned as part of a 1-2-3 combo
Though, the overall issue of 'losing your 1-2-3, cast times or not, due to interrupting casts, moving mid cast, etc' is moot, because if you were to miss, for example, the combo step you're on does not advance, nor is it lost, you just have to try that step again until you land a hit. I would imagine that a 123 that had cast times would be the same, it only advances you to the next step if you successfully 'trigger the effect of the move', which means interruption via movement, missing because blinded, all of that, would all be non-factors.
But again I default to 'why have 1-2-3 combos, cast times or no, when we could have the much more dynamic 'X ready' procs and SonicBreak style seperate-CD-GCDs, which IMO fit much more cleanly with the overall design of the healer role, being as it's meant to be the 'quick, react to this guy standing in the fire' role. Yes, the 1-2-3 won't get interrupted by you casting Solace, sure, but I'd argue the 'feel' of using 12/15/21/30 whatever staggered timer CDs, and filling with Glare etc, just 'feels' more right. Because you'd be able to make decisions with that kind of design, that you can't with a 1-2-3.
Take an example fight. At some point in the next 6 GCDs, Jimmy Samurai is going to get hit for a lot of damage because you can see he's playing greedy and does not seem too aware of the 'tell' of the boss unless it's a giant orange AOE. Let's say it's the trampling wildlife in Lapis first boss. With a design that has, eg, a 12s DOT and a 15s GCD, you have to decide: If Jimmy takes a hit when it's time to refresh the DOT, do you delay the heal or the DOT refresh? Same with the 15s GCD, which gets delayed? Does this lose you a use over the fight? If you choose to delay the DOT refresh, does this mean that both the DOT and the GCD are now 'ready' on the same GCD, and now you have to choose which to prioritize? And then, if it turns out the GCD that is 'removed' because you had to substitute it with a Solace is a filler Glare, yippee, nothing is misaligned, everything continues as normal.
Contrast that with a 1-2-3. You're planning to go 1-2-3-1-2-3 for your 6 GCDs. At any point where Jimmy gets trampled by the wildlife, you can simply throw a Solace in, such as 1-2-3-1-Solace-2-3. There's no additional thinking required, you just continue on after pressing the heal, as if nothing happened. And as such, it's hard to see how 1-2-3 equates to much more than just 1-1-1 again, without additional things like SAM's Sen or NIN/WAR's upkeep buffs. But those are presumably too complicated for healers to worry about. If they had existed in the ARR-SB times, they'd have been removed in SHB with the reasoning of 'we had data showing that many healer players struggled to keep the buff active, while also managing their healing output, so we have removed the buff upkeep and made the effect permanent, so healers can focus on their healing output more'
IDK I just feel like there's a lot more that can be done, in all design directions with 'X Ready' compared to the rigidity of 1-2-3s, bearing in mind that this is the healer role we're on about. Like, let's say you want to add a new action that is a hybrid of healing and damage. Let's say, a SGE action that does damage, and also applies one layer of Haima to the Kardia target. With 1-2-3 as the 'solution' opted for, you're stuck: it's part of the 1-2-3 (probably as the 3), and you have little to no decision making over it's timing. It's effect is just 'there', as is the case with Brutal Shell's shield on GNB. But, if it were on a 'Melainachole Ready' (example name) proc, there's more options. Perhaps the proc can stack multiple times, so you can 'save' uses for that multihit TB in... well, P9S, P10S, P11S, P12Sp2 all have one. Alternatively, if it's not a 'proc' but a standalone CD like Phlegma, Krasis, Pneuma, Toxicon (god forbid), maybe there's a way to get additional uses, that a 1-2-3 can't do (except spellspeed melds lmao). Toxicon gets more uses because of shield breaking giving Adderstings. The damage lost/regained ratio leaves something to be desired, but the underlying design is interesting. You could, then, have something like this:
Melainachole: 1.5s cast, 45s charge time, 2 charges
Do damage to enemies in a cone in front of you with a potency of <Dosis potency> to the primary target, and 50% less to all secondary targets
Additional Effect: Applies a barrier with a potency of 100 to Kardion target. Multiple uses of this skill will apply additional stacking barrier layers.
Additional Effect: When damage is dealt via Dosis or Dyskrasia, the recharge time of Melainachole is reduced by 1s.
This action does not share a recast with any other actions.
edit: this is 100% unrelated to previous stuff, but I just had an idea: What if, instead of actions who's CDs replenish over time, we had a new kind of action, where you had only X amount of 'use' for the fight, and after that it's gone, such that you want to ration it out? As an example, Flamethrower for MCH. What if it were reworked to be much stronger such that you'd want to use it in single target, but you only have 30s of fuel, across the whole duration of the encounter? You'd want to put the channel times in points of the fight where you know you can stand still for a bit
Alternatively, cooldowns with 'charges' like we have now, gapclosers, Gauss Round, etc. But, you have, say, 5 charges of an action across the fight, and it does not recharge naturally. But, you CAN earn additional charges by using another part of your kit. As a quick and dirty example, imagine a Chemist class, where you have your basic crap, Cure/Medica analogues, and that's your fallback for when things are stressful. Your OGCD suite will be what you'd want to rely on, as is the case for every healer (bar one notable exception) in this game, but it's made from several of these non-recharging actions, so for example, 2 stacks of Indom-copy, 3 of Lustrate-copy, 1 of '1200p total regen', and to replenish them, you have to 'mix' a new charge as part of your job's mechanics. Rather than being gated by simple CD timers, you're instead gated by how fast you can perform the mixing. EG, if there's 3 filler GCDs of equal potency, correlating to a red/green/blue chemical, and you have to use a certain combo to mix a new charge of one of the OGCDs, you could fast-track certain skills to come back sooner. If you know there's a lot of AOE healing needed coming up, you can purposely use only the combo that mixes into an Indom-bomb, and replenish that sooner, and then mix up the other combos later