Usual argument is that your solution also "works" if you replace racists with, say, abolitionists at the point in history when the general public consensus was that slavery is just fine (except with internet).
Enabling good ideas to spill out of their origin communities into general public discourse is the main point of "public square" and "free speech" concepts.
They can still spill out through activist work, they just can't be as easily astroturfed by troll-farms so people will have to come by their influence honestly, through moral suasion and persuasive powers.
Enabling good ideas to spill out of their origin communities into general public discourse is the main point of "public square" and "free speech" concepts.