I think the issue of excessive MP and downtime aren't really the same problem. Downtime is downtime, regardless of if you have MP or not. WHM was heavily MP starved late in Creator to the point where I was riding my BRD's coat tails and chugging MP pots on cool down to keep throwing rocks in A12S.

Rather a better approach to look at this would be to consider that instead of having too many resources, healers simply don't have enough facets to fill their time with. ASTs are halfway there with cards (And I made a huge suggestion years back recommending that cards get taken off cooldown and put on the GCD). WHM have pretty much nothing to do outside of healing or dps whilst SCH are about to join the WHM camp thanks to the decision to remove most of the fairy micromanagement.

Other successful MMOs have healers setting up combos in advance of mechanics or events, placing and maintaining buffs or even putting as big an emphasis on their positioning as their GCD efficiency. SE just aren't interested in investing the time and effort into implementing involving and rewarding healer gameplay which leaves us in the position we are.