The thought experiment is kinda redundant because the situation would never arise, the only definition of healer in 14 is “has a green icon”, WAR and PLD already do the healing role in dungeons, RDM can be an emergency single target healer if needed and mitigation is now mostly a team effort

The definition of roles in this game is so arbitrary because square hates the idea of one role being a fail point, RPR is a surprising good off tank in dungeons if the tank dies, and most “support roles” do more than 50% of a true DPS damage, now you can argue this is a good or bad thing (I personally like how much damage support roles do but I wish the roles were more distinct, like mitigation should be the tank and healers role, healers shouldn’t be invalidated in dungeons, a DPS shouldn’t have better rezz capabilities than a healer etc

But this all comes back to the age old MMO problem where DPS don’t actually belong in the holy trinity, DPS is something everyone is responsible for, however if you give DPS no responsibility and the most fun rotations who the hell would pick a tank or a healer