So presumably 70% don't.
If you're never going to earn enough to repay the debt .. one might question whether there was any point going in the first place, surely?
I think this might be looking at it the wrong way around -
Firstly, I don't think higher education should be intrinsically linked to financial benefit, as it doesn't give you a full measure of the quality of a society. To pick just a few examples: Nurses, criminal lawyers, bee keepers, midwives, social workers, paramedics are all careers which either need or can be enhanced through higher education but would probably never generate the financial wealth necessary to repay a massive student loan.
If we accept the premise that there are jobs in society that we want, but don't generate large income/tax revenues for the governent, the question then becomes, why is there so much cost placed on the individuals who choose do these jobs? I think its so fundamentally wrong that an 18 year old who chooses to train to become a nurse has to take on £50k worth of debt, pay for all their own housing, dental/optician treatments and gets no help from the state, but that same individual chooses to sit around and do faq all, they will have all of the above provided for them