Most basic example to explain reason would be; healer balance is not something like "healing is still possible/easy even if we reduce heal amount by X%" because it is not for 90% of healers in our player base.
We can't "balance" certain class groups easily if it is causing us to lose majority of affected players in the end. Class balance for any class can't be objective for every player for this reason as it is considering majority of player pool in end game content instead of a certain group.
But 90% of healers do not all run instance X, X being just for example dark core, dont they?
You did already gate endcontent by introducing gear score requirements, so you can expect mininum values for players that enter there - reducing healing by X in that specific instance shouldnt be an issue for the rest of healers then.
I did not mean reducing general heal amount by X, if that is what you understood
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VERY basic example, not properly scaled:
Dark core: reduce healing by 20%
rofl: reduce healing by 15%
inferno: reduce healing by 10%
..etc
assuming you make balancing decisions based on what works for 90% of affected players: chose decrements for each zone based on gear/values that 90% of players have who regularly run these instances. tracking wisdom values of players entering a zone isnt too hard, or is it?
And which majority of players would you lose by implementing diminishing returns for heal scaling, when too much healing done in endcontent is the issue?
If this isnt the issue, this discussion is obsolete and I have my answer: healing does not get progressively harder because healer players are not skilled enough. end of story.
Cheers!
EDIT: although I personally never entered dark core, form what I heard healing in rofl should be reduced even more than in dark core...