Why make it illegal? Why not offer only federal loans?
How about:
20 year loan, 4.125% fixed rate 30 year loan, 4.375% fixed rate
No early repayment penalties and maybe interest returned incentives for full repayment return before term at certain benchmarks.
The average debt for a 4-year Bachelor's degree is $34,700. At the end of 20 years the total repayment amount would be $36,131.375, 30 years would be $36,218.125.
Looks good to me.