If someone is making a consulting agency that gets clients, deals with most of the risk, etc., then that is worth something to me and I'd be willing to take a less then I could get if I were doing the same work freelance. I'd give up, say 10-20% of the value of the contract to pay for such services. If the owner wants to take much more than that then he/she will have to expect people to jump ship after training. The salary difference is worth taking on these other responsibilities yourself or finding an agency that will take less.
As far as your second example of people, I don't think enough people are like that to warrant consideration. People are more likely to stay with a company beyond the point of rationality.
The problem is that if you spend 6-12 months training people you've got WAY more costs to recoup than a normal contracting firm so you'd have to take a higher percentage off the top. The rates would be higher for the niche so the take home would still be good, but a smart person would see you taking 30+% off the top, and know they can only give up 10-20% at another place or none by going freelance once they have some network in place. The sunk costs of the training and bench time around that are very hard to make back.