A mixed audience can be a problem. You’re right they’re not dancing together but it’s better if there’s a mechitza than if there’s family seating. What several venues have is that there’s family seating and seperate seating as well if you want that.
A frum concert unfortunately nowadays can become a problem. Several years ago I was at a concert where the performers-it was a band had a really good song where the guys went up to the dance. They got into it, and then it started dying down so they started going back to their seats. Then the girls decided they wanted to go up so several girls went up and started dancing and making their own circle. (The concert was sponsored by a modern coed school so that was probably why they started going up. But still the performers were frum people and most of the people there were frum)The performers kept right on singing
Several years ago, a friend of mine was at a world famous concert (it made it onto CD/DVD) and left when there started to be mixed dancing.
Most of the time this doesn’t happen. But it can happen