While I do agree that AST will obviously gain more pDPS from these changes, I do think the Lightspeed change is being a little overrated. It will make some fights a bit easier, as you just need to smash one button and then run like a headless chicken, but generally in raids I have found that one Swiftcast and Broil II has been enough for me to position myself to safety and then continue casting. I can't think of any raids where you need to be on the move for that long, and where you wouldn't be able to cast in between moving.

WHM is popular because of its' straightforward playstyle (which I believe is a much more important factor than people give it credit for), as well as it being a powerhouse in progression. The AST changes are not messing with any of that.