This is a loan from the state, so the result is the state doesn't get repaid. Though remember the 83% figure is the number of people who won't FULLY REPAY – far, far less won't repay anything at all. Already graduates who started since 2012 have paid back £200 million and they've only been out of uni a couple of years at most.
Initially in 2012, the system was set up with the anticipation that under 50% wouldn't fully repay. They got that wrong, and now it's predicted to be far higher. It was 77% just before the announcement of the repayment threshold increasing to £25,000, and is now 83%. That means this has been nowhere near as successful for the public purse as it was supposed to be.
Yet it still does mean compared to before tuition fees, when the Government simply paid universities by a direct grant, billions are now paid back. Whether that's enough to justify the tuition fees, or if it's caused inflationary issues on universities' costs, is for the politicians to debate.
PS: I've one more thought on this, though it may make your brain hurt. The problem with saying the "taxpayer foots the bill" for what's unpaid, or "the state makes a loss if student loans aren't repaid", is that the whole concept of what SHOULD be repaid is arbitrary.
For example, one way to increase the number of people fully repaying would be to cut the interest rate to the rate of inflation (as it used to be before 2012). Do that and more people would repay in full within 30 years, thus the 'loss' would be reduced. Yet that would mean the total amount the state received would be less, so the taxpayer would actually foot a bigger bill.
Conversely and perversely, if interest rates were increased, and for ease let's be ridiculous and say to RPI + 100%, then almost no graduates would fully clear the loan within 30 years, so the 'loss' would be huge, but the state would take in far more, leaving the taxpayer far better off.