If ignoring them worked, we’d be fine.

In principle, I agree with you. But the problem is, these assholes already have the platform. They own it, in the literal sense (they outright own or hold large shares in the major social media platforms and the traditional media outlets) and in the more general, vague sense (their beliefs, even if fringe, will get pushed into mainstream discourse because of their grotesquely outsized disproportionate influence). He’s a billionaire. If he wants his message heard by the world, it will be heard, period, because we live in a capitalist society, which isn’t called a dictatorship of the bourgeoisie for nothing.

Given this reality that their ideas will be disseminated regardless, it becomes important to counter them. To shout out the truth that their ideas are disgusting and how they do great harm to regular people and to the world. It is important to mock them and show others how ridiculous and heinous their ideas are.

In short:
If an evil voice has no platform or is struggling to grow one, then I absolutely agree - do not give it to them. But when they already have it, it can only make things worse if their opposition (we, ourselves) remain silent about the harm they’re already doing.