Instead of nerfing Mercy ...

Buff other characters/supports.

Lucio can't heal tanks or vs high damage comps, even in combination with other healers. His only benefit is Sound Barrier and boops.

Ana can't heal maybe three but not handle heavy damage, let alone keep tanks up. Her only benefit is heal denial and Nano.

Zenyatta is basically DPS assist. His heals do shit. Damage is good, though. His only weakness is getting hooked/hopped while Trancing

Buff other supports, but keep Mercy the same minus maybe removing invulnerability & action loss on revives. Making you unable to do anything while being revived is basically a death sentence for some characters. Sometimes the revive is too good.