The biggest problem with the modern fights is the lack of dynamic mechanics and job kits that have diverse tools to react to those dynamic situations.
Neither 1-2-3 combos or 1-1-1 combos makes the combat interesting, they both represent a single choice in a larger part of a jobs kit out of what should be a multitude of choices. PvP works because of the dynamic nature of the fights requiring you to not think in a parse-brained manner where something has to be used on cooldown or it's going to waste, where it's worth using that weaker stun attack because the rest of the group is going to down this straggler as long as they can stay still. Jobs there only have a single hotbars worth of skills, but they require far more thought in their usage than PvE does. If you hated the idea of 1-1-1 in PvP, you could technically just copy the same action on the hotbar 3 times and emulate the PvE experience if you really wanted to, either way that isn't what makes PvP exciting, what makes it exciting is the need to make on-the-fly decisions that if made poorly, can cost you the fight.
PvE completely lacks this kind of dynamism, fights are incredibly scripted and lack any kind of random variance that could heavily disrupt rotations. There's only add phases about once, maybe twice an expansion, and there's no need to kite or CC them. Bosses don't need positioning anymore, they're strong independent bosses that don't need no tank and will decide where they want to be themselves, which is going to be the center of the arena where they'll put their static mechanics that will never change position outside 2 presets chosen at random. The other preset is possibly used for the second half of the fight.
Because of this kind of fight design, CC is an afterthought on jobs and only exists because of older raids that made use of them. Tanks are just melees with defensive cooldowns and don't have to make any concessions on keeping aggro, positioning, or their mitigation. Tankbusters are also not that frequent and so you almost always have a cooldown ready for when they eventually pop up, meaning there's not much need to think out cooldowns unless you're planning to optimize. Most DPS cooldowns are pretty unremarkable as they're just use-on-cooldown, I'm not making some super-smart decision on saving them for that special moment, I'm just wasting them by not using them. Gauge abilities can at least be saved for burst phases, so they're not entirely brain-dead.
Don't even get me started on healers, I've already spent enough time complaining about them on the healer forums. If 1-2-3 combos actually added any kind of depth and made a job enjoyable, you wouldn't see healers groan at someone suggesting to add them to their kits. What healers instead argue for are extra job-based mechanics; gauges, stacks, buffs, debuffs, procs, etc.
Okay, so what does this tirade about fight and job design have to do with combo consolidation? Because dumbing down comes from the removal of meaningful choices and interactions between mobs and players as described above, hitting 2 before 1 is something that occurs only when fat-fingering, it's not a meaningful choice that you consciously make, instead it's shoved into muscle memory during the leveling process. The choice to use the combo is meaningful, but it's a single choice split needlessly over 3 buttons. The only job that would be affected by this is MNK, which shouldn't be consolidated anyway since it technically has its own combo system and actually has good reason to use skills outside of their combos. Removal of positionals is a far better candidate for describing something as dumbing down.
If more buttons on a job somehow made it more enjoyable just because of the fact that it has more buttons, then the easy way to fix SMN is to remove Gemshine and Precious Brilliance and put the elemental abilities on their own hotbar slots. Do the same for Astral Flow, Enkindle Bahamut, Astral Impulse/Flare, Fountain of Fire/Brand of Purgatory, and boom, fixed, SMN now has 39 buttons. It doesn't change how it plays, it's still choose a colour > press the colour 3 more times > Demi summon > spam the only attack that actually lights up > repeat, but hey, it's not dumbed down because it has the most buttons out of any job!
SMN is bad because the choices don't feel all that different from each other and it's a very easy job. The only interesting choice you might have to make is "Can I get away with using Ifrit?". MNK I find more technical than the other melees and yet it has the least amount of buttons amongst them, so as far as I'm concerned, button count != complexity.
The reason I argue in favour of consolidated combos is because it's literally free hotbar slots, that's it. Jobs still have the same amount of choices they normally have, they don't suddenly lose out on any output, they don't lose any APM, and they still have to choose what combo path they have to follow down. It just doesn't waste 2-5 hotbar slots.