I think they designed themselves into a corner with the healer role. They had two healers at the start and they set up the healing dynamic time be you should bring one of each for maximum synergy. Each healer had a sub role, or style, of either a reactive healer or a preemptive healer.

Rhy they released AST. Instead of creating a new healer style, and balancing the combos of styles to be about even. They doubled down on the safe route. Created a healer that can sub in for either of the current healers.

Now they've created a precedent of there is only 2 healing styles. The next healer they can not use this safe route anymore. So a lot of work will need to be put in to properly Balance healing roles be use they've never done it before. The next time we see a healer, I belive it will have to be a single job release like ninja.