you haven't explained why we should want to stop people from doing those degrees.
I opened the thread supposing that you were looking for advice on how to dissuade someone known to you from doing one of these; a gentle intervention. Certainly I'd be keen to encourage second thoughts in a good friend, and certainly a child of mine.
In truth, though, you seem to suppose the argument here is about economic costs to the country, and I think it is not.
Suppose that Martin and Neddy go to Man Met to study Biomedical Science and Media Studies respectively. As anticipated Martin earns more than Neddy and he has paid off his full debt liability at 40 while Neddy dribbles it in until the debt is written off when he's 50, by which time he has paid off ₤15k.
The thing of it is that the ₤27k paid-off by Martin perhaps represents 80% of the provision cost for his degree, while the ₤15k paid by Neddy represents 120% of the cost of providing his. Because providing in Media Studies is cheap and in Bioscience is costly. For all that he hasn't met the whole of his liability, Neddy has de facto subsidized the cost of providing Martin's education.
And while at university they weren't claiming benefits or housing and were both available for poorly casual work.
That people are doing degrees which likely don't confer much benefit on them is bad news chiefly for them.