Blizzard is running into the problem they created the last time they tried call certain classes “hybrids.” They are making certain classes superfluous. A self sustaining tank, as you identified, is bad because they render healer’s role to top off the DPS. Adding more damage to healers would have the same effect on DPS. As a healer you’re necessary, but forever the sort of party maid, there to clean up a mess when someone has an oopsie.
When it comes to group mechanics, you either force people to stay in their lane, which is the trinity of healer tank and DPS, or you remove those lanes entirely. The second option isn’t particularly within WoW’s game design, because then it would more closely resemble Diablo.