You're right, just merging combos wouldn't work for something like BLM which is why I don't think it's the solution either. But I was personally never too fond of combos anyway because unless they are branching combos that give you a meaningful choice, they are extremely wasteful with hotbar slots and don't add anything to the gameplay. Pressing glowing buttons in order for slightly increased potency isn't the pinnacle of skill expression, no matter what some people claim. And your entire ARR is basically just "press 123 in order" with things slowly getting added during HW but you feel the extreme low level skill pruning in lv 70 Ultimates.
If the only engagement comes from "Well, you could accidentally press the wrong button in panic!" then that's really not much compared to, say, miscalculating mana ticks and being left with a short AF phase when that wasn't your intention or starting your melee combo at the wrong time.

Branching combos are good though and they're not wasteful either. And they offer a simplified but solid version of your rotation if you're just not at a point where you're good at doing them correctly. I was one of the people that just used 123456 on MNK when levelling it in ShB. Was it ideal? Nope. But it did the job well enough and I wasn't neck-deep into MNK gameplay.

Of course I'd personally prefer to have a good revamp with a close look at a lot of skills, the 2min burst meta that can go to hell and adjust it all but I know it won't happen.
FFXIV relies a lot on having almost 3 hotbars full of fluff and the sheer amount makes it seem complex but then about 1/4 of it is for aoe alone, 1/4 is "fluff/ niche/ you never use it lol", 1/4 is "flat potency press on cooldown" and the remaining couple of buttons are your core rotation - which then boils down to pressing 123 and something else in the correct order.
And I've seen in other games how you can engage a pretty engaging gameplay with just 5 buttons. And yes, that would work in FFXIV as well.
But quantity gets mistaken for quality here on many classes and the below current expansion gameplay gets increasingly worse.