When you take a percent of income, you necessarily demotivate the worker. It comes down to this: Once the bare necessities are paid for should I work an extra hour, or should I go home and spend some more time with the family? If the income for the extra hour is decreased, it necessarily decreases the worker's interest in working the extra hour. Or maybe they still work two extra hours, but not three.
To make this work out you either need to make the payback a simple loan (the traditional solution) or you need to put an achievable cap on payments. Either way the optimal societal goal is that if the worker works a bit harder they should earn 100% of the profit from the extra work.
Historically this worked out as the disaster that was sharecropping. The tenant earned 50% and provided labor, and the landowner (who also provided capital equipment) earned 50%. The natural outcome was that laborers rationally did not work as hard as they could have, and the landowner failed to properly invest in equipment since extra income from productivity gains was halved.
Percent of income may make investors salivate, but it's a self-defeating idea.