The bit with "have you seen the museums? it's mayhem in there!" is not a valid excuse. I think it's a shame that kids aren't taught manners very much these days, because in large groups like that is when you really NEED those manners to kick in. Otherwise it's a nightmare. We hate crowded, noisy museums.
I agree, just b/c it's crowded doesn't mean all manners need to go out the window. If I had a child that I know had a tendency to be pushy and rude I would be helicoptering over them and using this as an opportunity to teach them to use their manners.