This presumes that any decrease to present button count would necessitate that new button-consuming actions must fill the then savable spaces. While I wouldn't be opposed to that, it runs contrary to trends since Heavensward, which have usually seen increasingly less depth included per button and have been increasingly less accepting of higher button counts, even to the point of removing actual and more integral depth (e.g., Monk's ability to position once each per stanceless DoT period) to try to remain within a "comfortable" button total.
First, it's not a new system. We've had it since Stormblood.The dev team isn't going to design a whole system specifically for combo consolidation only to also prune out old abilities.
Second, so what? At worst, those who refuse to consolidate go from, say, some 24 buttons to 27. That's hardly damning to those who like their buttons or consequent finger drift.
Admittedly, though, if this was all left completely to player option, such could require some new design (or, just no longer purposely putting barriers in the way of macro functionality), but that'd also mean that a player could set their GCDs to go 1-2-3 and 1-2-4 as they do Storm's Path or Storm's Eye, respectively, even while not wasting any keys (with, say, Fell Cleave and Decimate simply moving around based on combo progress).
A new system could allow for that "hit different button per step" engagement, while a 1-1-1 or 2-2-2 for each respective combo would, in fact, be a 3-expansion-old system ready for implementation.
So which is it? The system you're complaining about wouldn't have to be developed, while any system that would need development could address your exact concern.
Then where is your first argument going? If there's little to no chance of SE filling that space saved with new buttons for those who use that option, how does that "option" break down into a "requirement" over increased button count?And please don't argue, "they'll be able to add new abilities if they consolidation combos!" Look at Summoner. Just... look at it. They had a new slate to work with; an entirely fresh canvas. Look how many buttons it got. Do you really think they'll give Warrior two full combos or replace the 6-7 buttons Dragoon loses? I guarantee they won't.
I don't mean to "got ya!" or whatnot here, but it can't be both, so which is our predominant fear -- that (A) jobs will get more complex and thus make the option no longer an option, or (B) that the option will remain as much an option (and simply that, with it becoming more obvious that there are no unique affordances to having those separate buttons for rigid combos, fewer people will feel inclined to favor that bit of extra memory aid over button efficiency and thereby depopularize something you prefer, even if it still remains truly an option)?