So I have to disagree. WHM does have PoM, but can no longer pair Thin Air with it, but it's still decent.
However, outside of the opener, assize won't hit burst windows unless you hold it. Which means you lose DPS from holding, Aside from being 110 potency less to begin with.
Misery is still 900 potency so it's going to be even more of a DPS loss to use it at all. Of course, if you couldn't avoid healing and then it happens to come up during a burst window, that will help mitigate the DPS loss. But it's still that: mitigating a DPS loss, not a DPS gain.
SCH and AST are trickier because they have raid buffs. I do think at least AST will continue to be favored because of that. SCH I think is a maybe, chain strat is good but not that strong and considering how hard Sage will hit (And it will be able to hold its version of assize for every burst window without losing DPS...), I think it's a pretty good contender.
In any case, if someone were to ask what healer they should play for good firepower and then ask if it's white mage, I'd laugh. Unless there's some pretty big changes before launch, it's going to be "Just play sage"