AST does by no means having healing capabilities anywhere close to WHM. The main reason they are driven out of the meta currently is primarily due to the fact that the current raid tier requires healing checks so low that can be met by solely regens, eos and sch OGCDs alone. A8S and A3S were perfect examples of WHM viability. Progression healing through J-waves was near impossible without cure 3 and even in full 240 there was close lines were being cut, in addition to the other 999999 sources of massive back-to-back raid damage. Unless fights are tuned back to this format though, WHM will be dead in the water in 4.0 unless they are given more forms of raid utility. Right now yes, all the classes are viable, but the current content leans more in favour of the AST/SCH meta.