To be honest, I don't find that situation particularly rant worthy. When I read the title, I thought that they must have cancelled you at the last minute.
Basically they asked you, you agreed but warned of potential time/commitment issues, and then someone else came along who knew the material already and presumably doesn't have a child on the way. They decided to go with that chap, and let you know within two days of the initial conversation. All seems pretty reasonable to me.
Saying that, the "we were doing you a favour" thing is face-saving nonsense. It's a shame, as you have proved your reliability to them in the past, but I wouldn't hold it against them. Also, is the other chap doing music for a living? If not, maybe he's cheaper