The possibility of death, to the average person, is deterrent. Not to most of these kids.
Beside that they will, in all probability, have guns of their own and start blazing away. If they "lose" round one with you, they will regroup and hit you again, this time with more gusto.
I agree with your statement but for these situations (flash mobs) I don't think it practical.
Cowardly response.
Recent videos of two and five armed bros invading an internet cafe and a hock shop. Clerk whips out handgun and fires back. In neither incident was one of the miscreants killed. In neither incident did they fire back. In both they stumbled all over each other trying to get out the door. They are doing these things for some easy money or for some fun with a little loot thrown in. They are not interested in dying for these things .Arguably, in both cases had they hoods shot back they could have killed the defenders handily but they panicked instead. They are not ideological revolutionaries or bent on some sort of tribal revenge that would render their own lives worth the accomplishment of their missions.