Don't think healers dps is stoping people from playing the role. The same can be said for tanks too, it is responsibility.
Both healers and tanks have high responsibility from their very 1st dungeon till endgame raids. If they screw up it is 90% chance wipe.
Once someone has enough experience with healing/tanking then they can squeeze in some dps. This comes later, not in early dungeons, unless you are over geared.
Not disagreeing with you but this doesn't stop people from trying out healing in dungeons at least. All the things you mentioned is mostly focused on at endgame where people mostly play jobs that they are confident with.
Also a very important note, both healers and tanks will underperform by a lot with other random tanks/healers.
Unlike dps, where they just do the same thing with any party. Tanks and healers have to work together to max both their gameplay.
You cannot find this in dungeons or extreme farms unless the tank and healer are pre-made or have very similar mindsets.