Why does Ashe's Q need to nerfed like this

I'll admit, Q'ing a tower does a lot of damage to it really quickly. but Jinx and Trist can kill a tower in the same time frame. Instead of completely denying this tactic, and ruining Ashe's ablity to take towers quickly if no one is stopping her, what if riot just changed Q so that Q does 70% damage to towers?
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