??? You completely missed the point... this is not OW where fights are over in seconds, skilled play can result in avoiding significant damage entirely, and any small amount of healing is just a pleasant thing to have.
FFXIV fights last for minutes, 10 of minutes in high end cases, with skill only marginally determining the minimum amount of damage you can take, and the amount of damage healed in these fights being many 10s if not 100s of times player’s health pools. So it doesn't matter what the numbers are, how many healers you have, or who else can contribute to healing; if you are below the threshold it is non-viable, and if you are significantly above the threshold it is a waste of potential.
From example: Let’s say the boss does 100 D/s, and using FFXIVs typical 2/2/4 setup...
- If WHM can heal 60 HP/s and SCH / AST can only heal 40 HP/s (to 'balance' their utility), then a group without a WHM is non-viable, and 2 WHMs is a waste of potential, leaving only WHM/AST or WHM/SCH as good combinations. Not good.
- If we increase SCH / AST healing to 50 HP/s (so WHMs are no longer needed), then since both SCH and AST bring significantly more utility, even a single WHM, much less two, is now no longer wanted (which is the current situation). Still not good.
- We can play around with any combination of giving other classes the ability to contribute to healing, but all that means is that if you don't take class A you then have to take class B, and if you do take class X, you then don't want class Y (assuming class Y had to give up something for the extra healing ability). Again, still not good.
Now let’s say we reduce WHM to 52 HP/s (so still slightly better) and give them some utility to compensate... now all combinations of healers are viable (with any two healers adding up to 100-104HP/s), all combinations bring useful utility, and thus all combinations are wanted. Problem solved.
Will there still be a 'meta'? Yes, of course, someone is always going to do the math, but now it is much more up to player’s personal preference, rather than individual encounter design and hitting a certain threshold, that determines what healers (and overall group combination) they are going to bring.
Note: It should also be pointed out that, also unlike OW, any healing threshold is going to reduce with gear, making any 'pure healer' less and less desirable as gear level increases relative to encounter level. This is however not an issue if all healers have approximately the same healing output and utility usefulness.