Okay, now what happens if we take my (admittedly) hyperbolic hypothetical add some realistic possibilities?

Factor in for the shooter's element of surprise.  In the time it takes for 22,000 people, many of them likely too young to possess a firearm, to take cover, gather their wits, get a sense of where the shots are coming from, and respond in any coherent way, many on the ground will still be injured or dead.

How many sharp-shooters in the crowd?  Any concern for those living in nearby apartments?

I would like to think that my suggestion of a fully-armed concert audience firing at a lone shooter in an occupied hotel in a major city would be considered by most readers to be madness.

But then, I've never lived in a war zone.