>I think it's a lot about economics, too. Choirs of any significant size have to be amateur. It's hard enough to do a 4 person band playing popular music full-time, with 20-30 people performing more niche music it just doesn't happen.

That really depends where you live. In Vienna you definitely get all-professional choirs of that size for instance.

Really? I only know opera choirs as professional choirs of that size, and they kind of don't count, since the members are also actors and opera singers (just without a main role in that particular production).

When I think about it, maybe there are also choirs were members are paid, but are only very part-time employees/freelancers?