Technology is not standing still. Tor isn't necessary. We're seeing the rise of distributed, federated, encrypted, and anonymous networks that take little more than an app install or website link. They are only getting more hardened against these mitigation techniques and the approach of "just shut it down" will soon become an infeasible solution.
My concern with big companies deplatforming political extremists that weren't in the public eye is that it almost validates their "they don't want us saying XYZ because it's true" points. Not that their political shit has any basis in reality, but when impressionable people see that they actually are being squeezed out of the internet, it leads many of them to conclude that their other points are valid.
Right now, loads of extremists are taking to Discord and other private chats to discuss their points and recruit people. Inside those tight-knit private groups, there's no possibility of a random passerby to stop in and offer a dissenting view. They see their discussions as the absolute reality of the world. Pushing them deeper into those groups feels far more dangerous to me.
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.