It has been shown over [1] and over [2] and over [3] again that deplatforming is effective. So yes, they'll still find a way to talk to like-minded people, but you're incorrect if you think there will be nearly as many.
You're coming at this based on the idea that everyone is rational and sensible and (re)evaluates their views based on logic. Our brains don't work like that; literally all it takes for someone to warm up to an idea is being repeatedly exposed to it [4], and that's before you even get into things like confirmation bias/pleas to emotion/peer pressure/etc.
Using this reason to censor ideas that you disagree with _will_ lead to censoring ideas that you do agree with. Power corrupts, and if people have the power to censor certain ideologies, that will be corrupted and exploited.
I’m not advocating for de jure censorship of Nazism. But a “power corrupts” hypothetical about stigmatization makes no sense — society writ large has always had that power and will always have that power.
And yeah, it sucks that people on the margins of that such as sex workers get hurt, which is why we need to push to change those stigmas. But the alternative is letting open pedophiles teach your kids, or asking Jews to defeat their genocide-preaching Nazi coworkers in the “marketplace of ideas”. So all things considered it’s a fair trade.