I think that's not entirely true.
Indeed droods are the best class for m+, but if you check on
https://raider.io/mythic-plus-rankin...ards/2#content
you will see on page 2 and 3 that we have Painer, a priest, or even 김쪼* as pala healer ( still no monks ).
Which means that droods are the best ( and just wait for the pvp essence if they are going to leave the instant full dot cast in pve... ), but high lvl m+ are also affordable for other healers ( also, is full of shamans as dps, so it's a great thing they can go dps and achieve top scores ).
Also, regardless the fact they play as dps/tank/healer, you will see all the classes in the top 100.
Not saying that the game is fine the way it is, nor that you could ever fix a class which heals with hots while do a decent dps, but it is a fact that if you put effort and a definitely good team you can achieve a good goal.