IMO one of the primary causes of the issue is the decisions they made with both the healer role jobs and tank role jobs. Being that the 2 roles combined almost always if not always are a smaller percentage of the player population than DPS SE decided to try to encourage more players to play jobs in those roles by making the jobs themselves overly simple by giving tanks more self sustain, excessive enmity generation, and an abundance of damage mitigation abilities. While giving healers an abundance of "free cast" healing abilities. They then paired this with dumbing down the game content to the point the changes they made to the jobs in those roles became unnecessary in nearly every situation. The only reason tank roles aren't making the same complaints as healers in regards to boredom due to lack of damage buttons is because their damage buttons generate enmity so they were given a skill rotation to use full time. All the while healer jobs within the role have very few skills that will do damage while healing.

I don't feel the solution to the issue is to simply give healers more damage buttons and a rotation. The actual solution would be for them to fix the actual game design issues in regards to combat encounter designs vs skill kits while also up tuning 4 player and normal mode trials/raids so that non DPS players need to make use of most of their skill kit during battles. They'd probably also see a larger amount of players at least attempting EX/Savage/Ultimate if the game provided a steady incline to that content instead of essentially being a flat plain with a large wall placed on it that people need to either climb over or smash through to proceed.