So here's my concern:

STEM fields have some pretty high attrition rates. At my university, for example, nearly half the engineering/physical science students fail the first year calculus sequence. Of the ones who pass, most get by with a combination of partial credit and managing to guess the teacher's password. We're talking about students who were in the A-B range in high school running into a brick wall once they hit college.

The expected income of an incoming freshman engineering student versus the expected income of a graduating engineering student are two very different numbers.

In general, there are analogues to this sort of idea of paying a portion of one's future labor instead of a fixed amount of debt. The US military runs several programs where they fund one's college education and the student has to commit to serving some 4-5 years on active duty. There are other government programs in a similar vein, where school is paid for in exchange for a some years of work. For example, there's one for teachers who agree to teach in low-income areas for a certain period.