I've played FF since ARR. I dont play a healer so I cant comment on their pruning or homogenization but what happened to tanks and many classes going into ShB appears to be the ongoing design philosophy: lower the skill ceiling and the skill floor as much as possible. Any friction felt by the player or any decisions they have to make that come from their class should be removed. Look at what happened to DRK or tanking in general. Aggro doesnt even exist anymore, its just something that happens while you do your very simple dps rotation.
Is Kaiten the most important ability in the game? Does it require incredible amounts of gameplay and planning to use? No not at all, but what makes gameplay fun or class design good is not simply the amount of buttons to press or the complexity involved, but the subjective feel of the overall kit itself. Kaiten requires some minor attention paid to the kenki bar that eventually becomes passive, as the rhythm of the overall rotation becomes second nature. Its main purpose (at least now that kenki is in surplus throughout the rotation) is to give you the feel of winding up for a big swing.
Many have argued that its simply a button thats pressed before every Iaijutsu/Namikiri but by that token, why do ANY damage buffs exist? Why not roll buffs into potency of other abilities. Heck, why do we have to press Hakaze to start every combo. We sure press that button a LOT, why not simply have one button for each sen and thats it. The value of a button press is not solely in its utility, because if utility is all that matters, then the perfect class design is a single button that plays your class for you or you can hold down. The very purpose of having a variety of skills and abilities that interact with eachother is for providing the player with "fun". If buttons feel bad to press for the majority of players its bad design, if they feel good it is good design. The evaluation need not be more complicated than that. The difficult part is in the class design.
As fun is subjective the best thing for the dev team to do is to make each class as unique as possible, give players as many different flavours of gameplay as possible. Try to raise skill ceilings as high as possible while lowering skill floors as low as possible. Does a casual player who doesnt research their class have fun? Do they feel rewarded playing the class? Does a player that puts time and practice and research in get rewarded with higher results proportionate to their skill? Do they get to make decisions in gameplay that feel good and improve outcomes?