The already radicalised don't go away, but de-platforming does help.
Sure, there are some alternatives that are harder to restrict and monitor, like if all the radicals move to private Telegram groups or the dark web or something.
However all the vulnerable people at risk of being radicalised are less likely to be radicalised if they're using a similar forum to 8chan but without all the white supremacist and other hateful memes and "jokes", that obviously have radicalised a lot of people. Even if most people just scroll past them.