Doing so would destroy the ability of non-establishment candidates representing salient issues, e.g. police violence in particular communities, from effectively running grassroots campaigns.

Only if money remained the engine of political campaigns. But if others can't raise money either, then I think that helps the grass-roots candidates.

People who already have money could just spend it directly in that world, couldn't they?

They already do in this one. I will admit it would give the rich more of an edge.