Complicating matters: all are arteeeestes, and far too educated for their own good. Getting bargaining units like this to see reality and accept less than the grand deal they are convinced is their birthright is almost impossible. And very few actually live in Detroit; many do not even live in Michigan. They could care less what their strike does to an already decimated city. It's all about them.
The scenario you state is mostly true but a major symphony like Detroit is a major gig and is basically a full time job (although it is not 40 hours a week, it is several days a week.) They likely live in Detroit, at least during the winter concert season, and probably all year. They have recording sessions and other concerts to give, not to mention college teaching positions and private students.