I go to the theatre very regularly and it doesn't happen at the ballet and the opera.
If you're unfortunate enough to go to the family performances for ballet then yes there's more prancing about by children in their seats and aisles but it's still (almost) watchable 
What we're talking about here is populist theatre (which I also love) - I can only recommend that you go in term time to matinees to get a more sedate crowd (no children, people much less likely to take a day off work)
I do think the cost of living crisis has really affected everything - when people are able to go out (they go out much more infrequently) then they really LARGE it up - they get drunk, they screech and are very pushy. It's the same at the football and at concerts.
We're being so screwed over, having to work so much harder to get even a basic standard of living that it's not a surprise to me that people act like twats when they rarely go out.
It feels like the last days of Rome in town at the moment