> Somehow just paying them the market rate so they stay

But your competitors can pay them more anyway, because they have not spend their money on training.

> a (shudders) contract with the employee to stay for X number of years

Literally shudders. Employer has no practical way to ensure adequate quality of work from their employee. Generally it is not a problem, because employer can get rid of those workers one way or another, but when you have spent money upfront... Shudders.