I think this is because the other healers arent as accessible. So not realy a reliable number in the first place.
And 6 years is nothing extreme for something this small (be honest, most players arent maximizing their role anyway. forums are always a minority of the playerbase). WHM works in most content without issues. Its just that those who want to take it to the limit simply cant, and have to resolve to other roles to do that. This is why its hard to fix, you dont want to upset the current players by changing everything.
Putting everything off the GCD has the side effect that the cast pacing changes with it, essentialy forcing those players to do more than before. Ideal fixes do therefor keep the GCD in place, although this is significantly harder to do because the GCD is what causes a big part of the issues.
Playing another role is a bandaid for some players, but not for the role. And those other roles are already somewhat diffirent to WHM, so that realy is not a solution for everyone.
