"There is very little cross over between theatregoers and the unvaccinated."
Absolutely, but I think the issue that concerns people is that Broadway is an industry that is dependent on tourism from all over the world. And all it will take is one unvaccinated, asymptomatic person to put 1000+ people a night in danger, depending on how many shows they take in while they're in town. Especially since we don't yet know how much protection the current vaccines give and how long it will last.
I imagine that there will soon be a cottage industry of right wing social media videos with people coming to New York and Los Angeles and elsewhere and bragging about going into theatres and restaurants unvaccinated or having their fake vaccine cards accepted everywhere and how they're sticking it to the libs.
But maybe none of this will actually be a problem. Airlines are another industry where people are crammed into small seats in close quarters with hundreds of others, and that industry seems to be handling things and recovering.