"In this one specific case, convicting this one specific person didn't destroy the whole movement."

Well, no; because they haven't gotten together the political will to outlaw AfD itself as a Nazi party.

This doesn't show that banning the speech doesn't work; if anything, it shows that being overly tolerant of Nazis...lets the Nazis stay around.

I’m not convinced we can legislate morality.

If anything, outlawing it may increase its credibility as an anti-establishment movement.