I find this quite funny too... there's very little difference about pressing the same key 3 times while it changes than pressing 3 different keys that people put right next to each other in the correct order.
Consolidating the combo systems would allow space to add more abilities to be woven in or more skills that interact with different stages of the combo.
Plus there is indeed a ton of buttons for casters too, for example... Why Seraph and Consolation aren't the same button? Why Dreadyrm Trance doesn't turn into Summon Bahamut, and then into Enkindle Bahamut? Ley Lines could turn into Between the Lines while active. Dream within a Dream could turn into Assassination.
Basically there are a ton of conditional abilities that are only usable after a previous ability is pressed, and those could really merge into the same button.