1. The amount that you "borrowed" is a fake number, though, with no correspondence to the true cost of the course and is non-existant for e.g. Scottish students in Scotland.
2. Your salary will (is expected to) inflate; the repayment threshold currently does not, so the "time value of money" has something to say about how much you repay. Someone whose salary remains constant on the equivalent of £40k will find their repayments an increasing burden.
The repayment threshold is expected to rise with average earnings so someone on an equivalent salary of £40k would be expected to pay back less in real terms than that calculation if wages rise above inflation.
The amount you borrow is not a fake number, whether it is related to the course cost or not, just as the price of a car is not a fake number even if it was cheaper to produce. It also includes literal cash payments to you.
2. Your salary will (is expected to) inflate; the repayment threshold currently does not, so the "time value of money" has something to say about how much you repay. Someone whose salary remains constant on the equivalent of £40k will find their repayments an increasing burden.