1) It isn't 'crippling debt' because you only pay on income over 21,000. So someone on a low wage job would pay nowt, and someone on 25k wouldn't pay much either. It's only when you start getting into the realm of 'good' jobs you have to make hefty repayments.
2) I would be in favour of targeted help for kids from working class backgrounds- maybe a one-off bursary upon gaining good A level grades to cover those initial expenses at uni- the first months rent, the first train ticket away from home (or petrol for the family car if mum/dad is taking you), your first text books,maybe even a cheap laptop because most students need those. The sort of things kids from middle class backgrounds take for granted that mum and dad will pay for.
I've no idea if anything like 2) exists, but if it doesn't it certainly should. That's not the same as saying all uni education, or even all uni education for working class kids, should be free.-If and when someone from the working class succeeds through uni, and earns more than the threshold, they should repay towards the cost of their education like anyone else.