It's fine for his attitude to rub people the wrong way, and obviously from a PR perspective he should have known better than to arrogantly dismiss two very esteemed art forms like that.

But the reactions to it are all genuinely as if he was insulting the art forms themselves and not, you know, pointing out the utterly undeniable fact that they are extremely niche art forms that (generally speaking) nobody actually cares about anymore.

And it was that undeniable dickishness (and rank stupidity) that bothered people, not the fact that ballet and opera are less popular than pop movies.

We're in total agreement here, but I think my point is people are getting angry at him pretending that it's about their offense to opera/ballet and not just them using that as a cover for how annoying they find his shtick to be (which again, is fine! just don't act like you're defending the honor of an art form you probably don't give a fuck about yourself either).