"Healers should heal" - i dont know but what and where ? What MMO does this ?
WoW = nope
GW2 = nope
ESO = nope
Lost Ark= nope
Black Deset = nope

Just to name some of the big one's and some of them dont even use the trinity system (tank, healer, dps). I dont know how or from what they draw the conclusion but no game i know of has the "healers should heal" nonsense. For a simple reason: Because its boring and bad game design.