However, I predict that Bernie would have been a poor President because good legislation cannot pass ideological purity because it is by nature a product of compromise.

I don't think this is true. He was able to get stuff done in the Senate, and despite his strong ideological standing I haven't seen any evidence he couldn't work with members of both parties. The other thing is that he wasn't as blindly idealistic as he was painted to be: I remember him saying in an interview once that, since you'll always get less than you ask for, you're better off asking for the moon.