As others have said this is completely ignoring that most melee dps have branching base combos. Take SAM for example. As the person stated you have 1-2-3, 1-4-5, and 1-6. They need to be separate because two of them have buffs that need to be maintained to them and you may need to alternate where you are starting on each pull based on the status of your buffs and how much and which Sen you have. So at most, you got rid of a whopping 2 buttons. Not much difference and tbh... I don't see much advantage of it. Like okay wooo. The reality of it is, all you are doing is earning yourself about 2 button spots top for no reason other than... you want to just press the same button. Hells PLD will only gain a single button spot because it too has a branching combo.

That said, if it is purely optional. Sure whatever, but I don't really see a need to use any resources on making it a thing.