Martin Lewis does not factor in the emotional benefits.
Some people do not like debt. We don't, so buy secondhand cars, phones etc outright and keep them for as long as we can. Not something we have spent time calculating but it will have saved us loads of money over the years and helped with mortgage affordability calculations.
We, with some help from GPs paid for DC. It meant that our car is now 20 years old, and that holidays were cheap. (Covid travel restrictions helped.) DC also helped by being frugal and spending our/their money carefully. (Coach rather than train, taking lunch/coffee into college, looking forward to a big night out rather than clubbing three times a week, cleaning the rental flat so you got your full deposit back etc.)
The reward has been that they started their working life with a clean slate. It has meant that DS has not had to watch his debt rack up when he took a six year PhD. It has meant that DD, who is in a low cost part of the country, was able to buy a house in her first year of working.
I find the argument that you should take a loan because if you end up in a poorly paid job you won't have to repay, odd. It would be entirely up to DC whether they choose to, say, undertake aid work overseas and earn a pittance. But 18 is too young to know, and it is then a long time till you are 67. Not taking loans, if you don't need to take loans, is simpler and lower risk. A DD might find themselves mid-life having to retrain because they find themselves as sole bread winner. Much better if they don't have the penalties of huge, and accumulating debt. Or DC might find themselves in a high cost area where every penny needs to be spent on mortgage or commuting, and that marginal income, otherwise spent on loan repayments, really matters.
Paying for DC did involve some belt tightening. DM was able to contribute as tax rules allowed her to gift money to GC for education out of income - and I worked hard to keep her out of a care home, a huge saving, and to rent out her main home when she moved to sheltered housing, allowing her the income needed. Having DC start their adult lives without debt has been worth it.