There are two issues that make "out of sight, out of mind" in this case not the absolute worst.

First, it makes it much harder for them to radicalize and recruit new people. There is a lot of circumstantial proof that mere exposure to fringe ideas can convert a lot of people, even folks we wouldn't necessarily consider "borderline". Look at YouTube's autoplay algorithm and various conspiracy theories like Flat Eartherism, or stories we've heard of Facebook moderators needing to get rotated out to avoid radicalization.

Second, in moving to less popular platforms, less dedicated, existing members are lost as they stay on the old platform for other content. Voat is a good example of how these alternative platforms aren't always self-sustainable. If more of the popular platforms took a hardline stance, it scatters the miscreants to the wind, where their numbers dwindle and they hopefully, eventually evaporate.

It's not a complete solution and requires too much cooperation between adversarial corporations, but it's a start.