There's no doubt WHM will be viable. The doubt is whether WHM will be desired by groups.
WHM will have no problem healing any content in Stormblood... but neither will AST and SCH. So when it comes down to all three healers being equally capable of healing all content (and they are), then people start looking at what else those three healers offer besides healing (and dps). AST has Balance (20% more damage for the whole raid), SCH has Chain Strategem (20% more crit for the whole raid vs the boss), and WHM has... nothing.
People need to stop worrying about WHM being capable of healing the content. That was never an issue. WHM's could completely ignore Confession, Lilies and Plenary Indulgence and still heal all SB content without issue. The worry is that groups will generally look for the most advantageous comp, and WHM just doesn't compare to AST and SCH because they offer nothing that amplifies the entire raid's performance.
SE should really give WHM a higher level trait that makes Presence of Mind affect the whole group/raid. Just this small change would give WHM the capability of boosting raid performance, while also not adding a new button to press.