That's OK. People do move to platforms like Gab, but those platforms tend to be echo chambers in their own which has a limiting effect on their popularity in addition to the limiting factor of their initial toxicity.

They're not really good for recruitment (because they're echo chambers that only appeal to people already invested in a point of view), nor do they provide much amusement from baiting other people with very different opinions; these two factors are in stark contrast to mainstream platforms, which provide an abundant supply of both.

Naturally, you get cross-platform raiding where people organize on one platform to attack another and then enjoy the spoils if their attack results in upset, outrage, bans of the target etc. But off-platform raids are relatively easy for platform operators and target populations to monitor, detect, and repel. Likewise, the extremist or highly toxic platforms that people do congregate on function well for a policy of managed containment.