I've wondered for a while about making a career out of genealogy, so it's very interesting to read everyone's comments.
I think the problem is that researching your family is a) easier now more records are online, and b) fun. It takes a long time to research someone's family thoroughly, and to make enough money from this, you'll have to find people who are willing to pay that much, rather than research themselves.
Researching whole families may be a dead-end, but if you become a specialist in a niche area, records which aren't available online, you might find people willing to pay for your research.