I would honestly prefer healers to not dps at all because they are taking kills from dps who could use the battle high/fever more efficiently, which will raise the chances for the team to win.
Also you underestimate the power of full healer groups. They were impossible to kill. Here's a vid of my group back then against a healer stacked group. We were stalemated for 5min. Against any non pug group that didn't know how to rotate knockbacks and keep 4 people in the flag, they would have taken the flag and be impossible to flip back for the rest of the game:
https://www.youtube.com/watch?v=SW9e5UYlUgg
That was with clerics nerf. Nowdays, healers have even better aoe dps, especially WHM. It would be completely imbalanced. And you have to consider that the more healers you have, the more divine seal raise you have. At a certain point, you can just loop divine seal raises.