1-2-3 abilities getting squashed down into 1-1-1 would free up quite a bit of space for melee's.
Why.are.people.so.against.this?

There's literally no situation where you would need to interrupt your combo/start your combo over midway or use an uncombo'd 2 or uncombo'd 3. And if there are situations, is such a slight optimization that it wouldn't kill you to do it with the new system. We would save a CRAP LOAD of buttons if combos were 1 button and would open up the floodgates for more oGCDs and job gauge mechanics. It doesn't make content harder or easier either, it's just quality of life, especially for controller players, which is ultimately the limiting factor. Besides, it would only affect melee classes. Mages would still have their individual spells to cast.