What is the problem?
There was only one group who did it in one content. Who even sacreficed the freedom of free class choice, for substituted the missing healer. They even had healer in Form of 3 paladin.
And, it is something, where they probally failed multiple times to.

Yeah, maybe it can suck, if the healer isnt needed that much. But, that is nothing that can truly changed, without make it bad for other roles or the whole group.

"Let make the Boss more dmg" = can easly create frustration. Classes with weaker defense could easily be one shoted or two hitted with that.
Pandaemonium first 4er Set, second dungeon: I died each time practicly instantly, when the Boss had the unavoidable Aoe, while i was a Black mage and had only around 80-90% HP (the barrier was each time destroyed instantly). In the new 4er Set is it the same by the spider. His net can be enough to oneshot a weak defense class, if he is not at 100% hp. And DD has lesser healing priority by healer. This Boss even has a phase, where he make big dmg or wipe the whole Party, if the healer dont start instantly with the heal (or create barrier).
Ok, this problem are reduced, once the player have the higher item lvl. But, the problem would be more, until the Player reached that point. I even had sometimes raids, where it wasnt possible to finish it (or needed many tries), because the healers was low in item lvl and wasnt able to keep up with the heal.

It is not even the case, that they are not needed. In dungeon are they important for the mob, before you reach the Boss. And like i sayed a little above, are they needed yet for the New content (esspecial second raid).

But, it is good, if they important is reduced.
It sucks, if the succes is based mostly about, if one or two Player (or 6 by the big raids) plays right. It reduced the responsiplity of the healer, if they fails.
I can not count, how many times the healer died by the end dungeons of endwalker, last year. It happens so often, that my tank Partner and i maked allready fun about it. And whe was mostly only able to finish the fights, because the tank was a paladin, who healed me or protected me for the big dmg attacks.
What is the Standard Action, if both healer dies at the same time in a raid, and the group dont has a raiser?
The group jump from the bridge, if the Boss is not at his last %.
First time Athena: 4-5 times had whe a wipe, because both healer died at the same time.
One time when i played white mage and did the 75er dungeon: whe was unable to finish the last Boss, because he targeted each time me and i was unable to survive his one targeted attack (or the instantly aoe after that attack). It wasnt possible, because one Player (the healer) played not right or was simply to weak/ had a class with less defense (that is even the main reason, why i like barrier healer more as pure healer, they prevent dmg instant of "remove" the dmg).