Ironically GW2 has to an extent reintroduced a trinity set up in its new raid.

Fundamentally the trinity is valuable for both balancing and game design. It gives structure that allows for role diversity that creates a sense of variety in a game. In GW2 when they removed the trinity, healing and tank roles became useless which for people who enjoyed that style of gameplay greatly weakened the enjoyment factor of the game. A lot of people don't particularly enjoy dpsing.

The reality with gaming is the optimal will always rise to dominance, particularly in group content. Using a less optimal class by definition makes content harder for the others your playing with and makes you appear as a burden to be carried. Thus without a structure that allows for a diversity of strengths and weaknesses, you will simply have a handful of optimal roles rise to the top.

A support role is an interesting possibility but for it to work in the games meta it needs to be strong enough that you want to bring it. That also makes it something your expected to bring. For this reason, if they wanted to introduce a support role, they would need to release multiple support roles at once or convert existing classes into a support role (ie. BRD, MCH or AST). Otherwise you would have a Job that every group would need, regardless of if anyone in the static actually likes playing the Job.