Mozart actually had a lot of commercial success during his lifetime, mostly do to his operas.

As far as I know, Bach worked in the Fuedel system, where you were hired by a wealthy family to compose music for daily events. You where housed on the premesis, and pretty much acted as a servant, except your job was to produce music. But I'm sure if they needed someone to chop firewood or help out in the kitchen you were called upon. I know Bach had a large family and probably choose this way of life for its stability and security rather then be a freelance composer/musician/educator and work on commission like Mozart / Beethoven / Paginini etc.