First, we're not talking about any random group of white supremacists. We're talking about literal Nazis.
Second, the slippery slope is a fallacy. Literally. The "slippery slope fallacy" is a textbook fallacy. In this instance, many countries make it illegal to espouse Nazi ideology today, yet Germany and France haven't become dystopian dictatorships.
"If we outlaw gun ownership for the mentally ill, that's a slippery slope to outlawing all firearms."
"If we outlaw same sex marriage, that's a slippery slope to outlawing all marriage."
"If we outlaw cocaine, that's a slippery slope to outlawing beer."
All of those make just as much sense as a slippery slope of outlawing Nazis.
Second, the slippery slope is a fallacy. Literally. The "slippery slope fallacy" is a textbook fallacy. In this instance, many countries make it illegal to espouse Nazi ideology today, yet Germany and France haven't become dystopian dictatorships.
"If we outlaw gun ownership for the mentally ill, that's a slippery slope to outlawing all firearms."
"If we outlaw same sex marriage, that's a slippery slope to outlawing all marriage."
"If we outlaw cocaine, that's a slippery slope to outlawing beer."
All of those make just as much sense as a slippery slope of outlawing Nazis.