Symbols retain power for a long time. That's why we're not seeing a swastika revival in interior design or anything like that, even in places where the Nazis did not exist. Another good example is the U.S. confederate flag, which only existed officially for a few years and was decided against by a bloody and brutal military loss. However, it's retained enough significance that someone just pranced through the U.S. Capitol with it. If it doesn't have power as a symbol, why not pick a new flag?

Obviously banning a symbol is not going to prevent future violent movements by itself, but it also seems wise to not leave still-hot embers of symbolic power lying around for the next would-be demagogue to fan back into a fire. People trying to pick up those old symbols are giving a very strong signal about their ideology.

I agree that the main bulwark of a society against falling into fascism is its people's memory of the past and willingness to prevent recurrence of such episodes though.