Adding more DPS buttons is just one of many possible solutions. Others can be creating short-duration buffs or debuffs applied to targets, retaliating barriers, etc. Or it can be achieved without touching healers at all, instead changing encounters, creating new engaging healer-specific mechanics, and so on. There are many possibilities for how to fix the Healers problem, there are thousands of pages in many threads across the 5 years.

Also, I tried to write a more detailed answer to Nanami-san, that our goal is not just to add more attack spells, but my Japanese is too rusty, and the friend who can help me with that is currently asleep cause it's 2:50 a.m. in Tokyo.