I wasn't thinking about economics, that to me is not the issue. Both opera and musicals are examples of musical theater, period.
Now musicals are performed in opera houses, but a while ago, they weren't. A hundred years ago, operettas weren't either, and it even took Porgy and Bess forty years to make it to the Met.
This process of change is inseperable from history, and economics is a big part of that.
Even though they where bastions of exclusivity in the past, opera houses are on their way to becoming multi-purpose venues. In the old days, if you weren't noble, you had to make an application to go to the opera. Now, the tables have turned, they are desperate to keep out of the red. They are happy to take the money of anyone who is paying.
The young Boulez said that opera houses should be destroyed. I wouldn't care much if opera as an artform disappeared from the face of the earth, but I think that the buildings should be put to good use.
This has already happened, Carnegie Hall is an old venue which has hosted all manner of shows for a long time (e.g. Benny Goodman played there in the 1930's). During the 1960's it was saved from demolition by Isaac Stern and others.
The other thing is that even though they're classed as lowbrow, operetta and musicals help add to the performing repertoire. Let's face it, its dominated by a handful of composers roughly from Mozart to Puccini, and it really starts to thin out after World War I.
Whatever the theorists say in their ivory towers, the only constant is change, and money is a huge part of that.