When you call the fascists what they are, it gives them an opening to derail the conversation trying to quibble over the definition of "fascism."

Sometimes it's worth debunking their No True Scotsman fallacies, but other times you accept their "alt right" euphemistic synonym because it's more important to preemptively minimize their trolling opportunities.

Tldr: alt-right is a way to pretend this is a new political trend that has appeared in the US.

In fact, it is neo-fascism, i.e. a well known political trend that is built on the refusal of parliamentarian, pluralism and the rule of law. Their aim is a régime change to establish a violent government that would oppress anyone who stands against them.