There's evidence that deplatformining decreases extremists' reach and may curtail recruitment opportunities. It is an active area of research, but there is nothing to justify the position that "deplatforming rarely solves anything".
If we were to go the other direction, we can see how having a greater platform would be worse. E.g. if there were a blatantly neo-Nazi cable TV channel in everyone's home, we would expect many more people to end up watching neo-Nazi content and some of them to become radicalized. Propaganda requires a platform to be effective. It is thus not exactly surprising that you can make propaganda less effective by eliminating the reach of the platform.
If racists like those on 8chan can be pushed into the deepest corners of the dark web, where you have to use Tor to get to them or whatever, that's a win. Lots of people aren't going to bother, and thus will never run across them, and never have the opportunity to be radicalized by their propaganda/content.