Again, if the players with the GREATEST amount of downtime and who are LEAST taxed by managing it, and therefore require the MOST resource to appease would feel engaged, it stands to reason that there'd be more than enough for those who need less.
Again, I'm referring to those PLAYERS who are able and have a desire to do content like Ultimate. Even they should feel quite enjoyably engaged even in Savage and Extremes (and those who'd only care to go as high as Savage in Extreme and Normals and those who only go so far as Extremes... etc., etc.). No part of what I said limits the desire for healers to be engaged to just Ultimate.
You asked me to exemplify what I meant by a "skilled" player, so I gave a couple quick illustrations and gave the implications for those illustrated levels of player, while just being realistic with the bounds of mechanics they'd care to play with, regardless of what role they make take on.
An Ultimate-loving DPS or Tank isn't likely to be engaged by Normal raids, so this would already be to say that healers can and should be redesigned to have a broader span of engagement than either of those roles, and I would say an Extreme is at least as far above Normal Raids as Ultimates are above Extremes. For those "(at least) 80%" of players... who don't do Ultimate... that'd leave you with everything except Ultimate as sufficiently engaging.