I think they should be written off after a certain amount of years service with the NHS
It is. Most nurses will pay back hardly anything of their debt and it is written off after 30 years, ie early 50s for most graduates.
Someone one £30k pa, so a nurse with a few years experience, pays back less than £40 pm. Only the very highest paid nurses will pay off all their student debt, ie nurse practitioners or senior managers etc.
Most who spend their entire careers as band 5 or 6 will pay back not much of it.
Ideally graduates who work in the NHS should not face the same student loan costs as people in the private sector, but the reality is that very very few of them in lower paid grades like nursing actually pay back more than a fraction of the frightening headline £48k debts.