I do think that theatre actors are underappreciated and grossly underpaid. Film actors are far more adored. In my opinion acting on stage is more difficult that acting in screen because on stage, you are performing in front of a live audience where there is no place for mistakes and if you make one, it would ruin the whole performance whereas with acting on screen, if you get a line wrong or something, the director can just yell "CUT!" and start the scene again. I know both must be really difficult but I think theatre actors need to be acknowledged more for their extra hard work and not thing stage acting is like a step down from film acting.
Theater doesn't bring in the money that other things bring in, even though it takes a great deal more effort in a lot of cases, as it has to be done live with massive preparation and little margin for error.
But if the theater show doesn't bring in a lot of money, how can you pay them more?
So perhaps underappreciated, but hard to argue underpaid.