The issue with having multiple actions to "Equip" is then trying to balance jobs around having an "Optimal" set up without people who don't follow that being shunned or kicked for groups.

Meanwhile, FFXIV actually has a system that bypasses the issue of button bloat, in that it has the Trait system that allows them to "Upgrade" skills as you reach specific levels.

The trait system could literally be used to keep level 1-50 skill advancement intact, while merely upgrading skills into "New" ones later. It doesn't necessarily have to be in the form of Stone becoming Stone II becoming Stone III etc. As we have precedent of skills changing completely (For example, at level 70 WAR's Berserk becomes Inner Release and gains additional functionality)

This can also be used in addition to single button skill cycling. So instead of needing 3 buttons to do your 123 combo, you simply need a single button that swaps between skill 1, skill 2 and skill 3 depending on how far you've progressed into the combo. This further increases the room for additional skills to be utilized within a kit by allowing 2-3 skills take up only a single button of action bar space.

So long as they adjust gameplay correctly so that for example, Paladin wouldn't be spending levels 1-60 pressing 1,1,1,1,1,1,1,1,1 forever because of lack of any other skills to use beside their single combo. It would open up plenty of space for additional skills in the future without either messing with lower level gameplay nor with end game balance.