In Australia you have a loan balance based on your degree etc and pay it back with a similar % of income. I assume in the UK it's the same, if you earn 1mio you will probably not have to pay 9% (or do but can claim it back) and be done with any student loans.
Of course, but this isn't relevant to the consideration of whether UK student loans being are equivalent to an income share agreement.
If the point were relevant, then we might as well consider government spending on K-12 schooling the same way.
But general taxation is obviously not what we're talking about here. We're talking about the extra money someone pays in return for the government supporting their degree.
Does the government get 9% of that? Or is it capped at the outstanding loan amount?
If the latter, and someone earning $1m pays the same as someone evening $500k, it doesn't seem like an income share agreement.