The reality is that people don't want their Job to be the less useful one and the problem with a pure healer focus is that once you can heal something, any additional heals you can pull off is wasted. More to the point since even generally you have setups with one off healer and one main healer, assuming AST can still pull its weight as a Main Healer, its heals will be the amount you really need to do the content which will make any additional heals WHM bring wasteful. AST on the other hand will still bring its buffs.

WHM might turn out to be great and they might have solved much of the issue with healing. However I haven't seen anything to convince me they are actually addressing the issues present and I don't have much faith the devs have really come to grasp what the issues with healing design is at the moment.