The trinity system continues to dominate design because the MMO player base continues to be dominated by those who want the fastest, shortest, easiest path to anything. Take vanilla WoW for example and the frequent need for polymorph and rogue stuns for CC. After a while players started to complain it slowed them down and just wanted to faceroll through instances faster and faster to get their p#@+ 73w+z. As time progressed more and more CC opportunities were removed and players attempting to use it were shunned.

Fast forward to now where players still complain about how long things take and how hard they are so developers will give them what they want and continue to churn out cookie cutter tank, heal, DPS content designed, crafted, packaged, and sold to put Happy Meal smiles on a chicken McNugget generation of gamers. There are a lot of CC/support interested players out there, myself included. I don't find the role dull at all. But, like the cash shop, so long as there is a majority of players that don't want to mess with the deeper complexities of encounters and just like 'make boss HP 0 before mine' design, we are stuck with the trinity for a while.