While a raid buff would be awesome, it's not even the biggest bit of design clunk on WHM. Adding a buff on a cooldown would be a trivial fix for that complaint.

One of the major reasons people are predicting SCH/AST are still the most optimal is that their damage and heal kits flow together better. What I mean by this is that they don't have to choose between doing one or the other. They can do both at the same time. Fast cast or instant damage spells with powerful off-global heal spells let them keep going nonstop. WHM's "new" healing spells are the same as their old ones: the opposite of this. You need to stop doing damage in order to cast them, and the only damage ability you can weave there is Assize. Additionally, WHM's off-global healing kit is very anemic compared to AST and SCH, and this hasn't changed. Due to this, WHM has to resort to their global kit more often than AST/SCH can. If anything, it's gotten worse because the other two got better off-GCDs that they can weave with impunity.

Nothing about content design is going to change these facts. Even if outgoing damage increases. Think about it from a basic level. Which set of GCDs flows better?

(Attack, heal), (attack, heal), (attack, heal) (heal)

Or

(Attack), (heal), (attack), (attack, heal)

The above is the largest discrepancy between WHM and the other two healers at the high end. AST and SCH are getting a lot less fun by many accounts, but the core reason for the power imbalance is still there.