Yep, that's a very good point. Radicals need to have equally unreasonable, extreme views to justify theirs against. The only way they can convince ordinary people to go along with them is to say, "but look at these lunatics!!! If you don't side with us, they'll win!!"

In that sense, moderates are a bigger threat to radicals than their opposite extremists. Because we show that there is another way to think that ordinary people can get behind, but doesn't accomplish the extreme aspects of their goals.

It's interesting to see, on all kinds of issues, how much harder extremists attack moderates than they do their ideological opponents.