I think you are conflating 2 distinct financial decisions:
- is it a good idea to borrow interest free on a credit card and earn interest /investment return on the money
- is it a good idea to fund your child’s final year at university yourself instead of taking out additional student loans
The risk of (1) is that you lose or are tempted to spend the money and then can’t repay the money borrowed on the cards. It sounds like this is unlikely. In which case: fill your boots!
The issues around (2) are complex because there are lots of known unknowns around your DC’s future earnings. I think in one of his articles on the subject, Martin Lewis suggested that parents considering paying their children’s uni fees might hold off until their child got their first post uni job as this would give them some indication as to whether their child was going into a high earning career or not (although they could change careers later!) The interest incurred could be regarded as the cost of this option.
The IFS (https://ifs.org.uk/articles/student-loans-england-explained-and-options-reform) estimated that 79% of students on plan 5 loans would pay off their loans in full. (Some govt figures suggest a lower figure of about 60%). So only 21% would have any loan+interest outstanding after 40 years. You’d need to dig into their figures, but some of this 21% will have paid off almost all of their loan. - maybe only have a few £100s left outstanding. It’s quite difficult to conceptualise as we’ve all heard the mantra “if you don’t earn enough, you won’t pay it back”, but most people will pay it back in full, even if it takes them 40 years. Interest accrues at RPI, so in theory in real terms you only pay back what you borrowed but in cash terms there’s a lot of interest.
Also there are unknown unknowns: the govt may change the repayment terms of the loan. Could they increase the interest? Yes. Could they extend the repayment period? Yes. Could they change the repayment threshold? Yes. These are not “normal” loans!
There is also the simple cash flow benefit for your child when they stop repaying the loan earlier than they would have done. There is also the psychological benefit of having a lower debt figure. Do you/they value these?
It is also a way of passing on your wealth to your child sooner rather than later. This may/may not be relevant to you. So even if the overall effect of you paying the fees now versus them paying back the RPI-linked loan is neutral, you will have avoided IHT on that money. It may be more useful for your child to have the money towards a house deposit though.
And finally, a side point but addressing one of the points raised by another poster about women being less likely to pay back their loans because of lower earnings, time off for child rearing, caring, working part time etc. I’m seeing less of this in family members currently in their 20s and 30s than in my generation. There seems a much stronger desire (need!) to return to work - and work full time maybe with some WFH - from maternity leave - not wanting to lose out on pensions, promotions etc. And also needing both incomes. This may not be universal, but I would be cautious of making the assumption that women are less likely to pay off their loans in full than men.