I don't think you were unreasonable. People saying nothing just reinforces teenagers impression that everyone is in awe of them/ too scared, which in turn makes their behaviour worse. If my dd came home aged 16 and told me a woman swore at her and threatened her, I would definitely take it with a pinch of salt, and I would also be glad that someone had had the balls to set her straight, seeing as how I wasn't there to do it!
As for swearing, well, I don't really have a problem with it, it's obviously a language this girl understands, why would it be intimidating. And if it is, it might make her think about how her own behaviour might have been intimidating. Which may, in the long run, actually do her a favour. As far as I can see, the girl stopped her behaviour, and learned a valuable lesson (even if it is just that there is always a bigger fish, so keep your head down!)so everyone is a winner (including the street performer!)
I had a similar situation with a couple of really hard looking teenage girls while waiting in a queue for a ride at a funfair. They looked at the long queue, then asked the woman in front of me the time, then just slid into the queue beside her! Everyone was chuntering, but noone was going to say anything, including me, until I heard the biggest one say under her breath to her mate "See? told you! Losers!" At which point I tapped her on the shoulder and informed her that yes, amazingly I HAD actually noticed her crass and pathetic attempt at queue-jumping, and that the only reason I wasn't kicking her ass right to the back was because I had been brought up better than her! But she needn't stand there thinking she was so clever she had got away with- rather that the rest of us were too polite. She went bright red and....let me go in front of her (everyone else continued to chunter, though!!)