The reason why this sort of nerfing paradigm is inherently flawed is the fact that we aren’t limited to exclusively 2 healers in teams. Instead of addressing the actual core fundamental issue of her kit which was the rework (res on a 30 second cooldown timer that is UNEARNED, UNDESERVED and requires no skill to EARN it).
You can break off her wings during ult and make her fly time even 5 seconds. You can flat out remove her damage beam and her pistol (lol pistol meme). You can make her heal 30hp/s. But she will still be mandatory because of her revive that is unearned and undeserved and freely given.
So right now we lost 17% of our healing. Painful by any means. But this doesn’t change why we bring Mercy over Moira who always had higher HP/S from secondary not including orb (80) and AOE healing to boot through all 3 of her healing methods.
What we will see coming up soon is the rise of TRIPLE SUPPORT META.
Zenyatta can bring a lot of damage and add healing including his transcendence the only thing to try and stop the current abomination of Zarya + Hanzo ult spam.
Mercy will remain the rez bot. Mandatory because of free unearned 30 seconds revives.
Now insert Ana/Moira/Lucio/Bridgette as you see fit.
THIS IS WHY THESE NERFS MEAN NOTHING. We flat out need a re-rework or a revert. We will adapt and mess up the entire system of balance with this sort of nerf. We will see triple support become meta.
Until that point Mercy will not and can not become anything less than mandatory.