Gonna be the devils advocate and say that in a vacuum turning combos into 1 sequence buttons is neither good nor bad. It depends on whoever is working with it, and the actions, the jobs whole toolkit all around it - in this case SqEx.
And for the most part, I don't believe it's in the right hands. Over the last 6 years, I've seen them removing more actions/buttons than adding into the game (aside from PvP). Take MCH's removed actions e.g. Rapid fire, Reload, Quick reload, Blank, Heartbreak, Bishop, Gauss Barrel, Dismantle, Arm graze, Palisade (Refresh and Invigorate), and for that we got: Drill, Bio blaster (same CD, could as well be just 1 action), Chain saw, and Auto crossbow... MCH has one of the most empty hotbar of all jobs. SMN was another victim, but I don't think I have to list everything here, you get the point.
1 button combos in itself is a great idea, and would open up slots for more actions with more flavour or gimmicks. But I don't trust SqEx to put in enough thought to create something interessting! Ofc they don't wanna make use of it, otherwise it would make them seem incompetent. Remove 2-4 buttons, but add nothing of value.
However, I don't agree with their reasoning. If merged combo buttons would be too simple and easy, then why tf do healers (and SMN kek) have 3-4 damage buttons? 1 dot, 1 single target spell, 1 aoe spell, maybe 1 other for flair. (And yes, SMN is probably second just after healers with 2 constant pressed spells, 1 occasionally Astral Flow, and the rest is pressed once every minute) For most jobs, it would free 2 slots. On NIN e.g. I wouldn't mind having 2 free slots, on WAR or MCH on the other hand... not so much, they already feel empty as they are.
In a parallel world combo sequence buttons would be a hit, and a great addition to the game to fight button bloat. In this world however, we are struggling to even fill our hotbars.