I am not asking whether it's good to design for a different range of players with different abilities. That's obviously true, and FFXIV obviously does this, unless you think EX/Savage/Perfect fights are only trivially different from lower-level content. I'm asking whether it's good to design around players who've "cracked" the game, because that inevitably makes it harder for healers who haven't gotten there yet, and because players will crack *anything* if you assume sufficient effort and time.
Healing downtime occurs naturally, and it occurs less when you and your party are less skilled, and it occurs more when you and your party are more skilled. Up to a certain point, healers are learning to use more of their kit--after that point, healers are learning to use less of their kit. This is not really true of other roles. (it might be true of tanks too, if aggro management was more of a thing and still more of a party responsibility) The more you design around healers who heal less, the harder you make for healers learning to heal more. Devs should account for as many cases as they can, but a balance has to get struck at some point. We can talk about what an appropriate balance is--I'm not saying the current iteration is the right one--but I don't think you *can* balance by assuming the specific cases where a job's primary role has been solved.