Mercy has been nerfed before, yet people still use her. The other support just need a boost, THEN they'd be played more. For example, Zen's movement speed desperately needs a buff. If Sym's turrets became healing turrets, then she'd be used more, etc. Nerfing tf out of Mercy won't change the pickrates of other supports that suck in comparison to Mercy. In fact, the only medic that's just as good as Mercy, is Moira.