Well I would say the success of a support based business would depend on the "quality of support / price for support" ratio. Justin provides extremely high quality support (answers questions quickly, even dumb ones..., has extensive knowledge of his platform and wordpress, etc.) for an extremely high quality framework. He happens to provide all this great support at a very reasonable price ($25, especially when you compare it to $50 - $100 or more for premium themes - and his is a flexible framework). I'm not sure what I'd consider a good score for this ratio, but the point is to just know your potential to provide great support and price accordingly.
I do like your idea about charging for child themes. I would pay $5 - $30 for a child theme based on complexity if I knew it was on the framework I like and someone else can execute the look I'm going for better than me.
"Learn something new every day to make up for all the things you forget."