I can see this working in Canada (or other states that want a boost in educated citizens). Subsidize education via forgivable loans, then forgive the loans if you work in-state. The main issue I see is that the US has a terrible history with loan forgiveness. You always hear stories of (eg) teachers who should get their loan forgiven being rejected for some paperwork issue or simply bank fraud.
Taxes are much like indentured servitude. We allow the government to do things that we wouldn't allow private entities to do.
> Or loans?
Loans for specified amounts are legal, subject to some fairly strict protections (limits on APR, discharging them in bankruptcy, lots of required disclosure). You can't generally write a profit-sharing agreement that way - how would you tell the person how much they're going to have to repay? How would you stop them declaring bankruptcy immediately to get out of it?