"this should come out of a central pot, not be paid by a student themselves."
The central pot isn't big enough given the vast increase in the numbers of students at University. So the Government is faced with two choices.
- Increase general taxation to pay for the extra students.
- Get students to somehow pay for their education
And the problem with 1 is....
"Especially since that student may actually go on to earn considerable less than the student they are subsidising."
The general tax payer will probably be earning less than the student they are subsidising, so hiking up tax to pay for those students has been seen as unpopular.
And then we come back to...
"why shouldn't parents minimise their children's debt?"
Because having gone with option 2 if you allow rich students to now take out the "loan" and repay the "debt" (as others have said it's not really a loan, it is a tax in all but name) then they duck out of funding the system. So the rich don't really pay for it but the "normal" will.
So the only way to allow paying up front would be to charge those who pay up front a massively higher cost than just the tuition fees and for the "loan" system to get a fair slice of that cash so that it can use it to fund other students who won't pay as much back into the system.