What is “most hitscan heroes?”
Surely you don’t mean mercy is a great enabler to sombra, reaper, mccree and tracer?
Mercy pairs well with SNIPERS, because they can take high ground and are harder to heal for other healers as they often also play out of their line of sight or range. This includes Hanzo who’s a projectile sniper, not a hitscan.
A ranged healer would not be a requirement to heal ranged characters if they literally weren’t accessible to healers like Moira.
It just so happens that widowmaker and ashe are the most broken hitscans. And it just so happens that whatever counters them, has been nerfed down to the ground, so the only counter is a mirror. Mercy could try to enable any other hitscan and it would not be even half as strong as a single widowmaker or ashe. Mercy is not broken, they are. They never needed mercy or her damageboost or her heals to be broken, as evidenced by Brigitte completely taking over Mercy’s role but Ashe still being meta.