OK, I borrowed ten thousand dollars on my "credit card" for an operation. At that time, I had an income of $50,000 per year, I was thirty years old and they figured that I would work another fourty years. So the credit card company figured I would have two million dollars in earnings over my life. So we agreed that I would pay 0.005% of my income back to the credit card company. I would hardly notice that.
Note that we don't have to worry about interest, inflation, deflation. All that would be taken care of by the growth in my income with time, which would include something for the general productivity of society and the economy.
I come into ten thousand dollars, I sell my patent, I get an inheritance, or I just have a stretch of time whmen I have few expenses and can save up ten thousand dollars. Do we need a mechanism where one pays back the loan.
At first I thought we should. But why?
The credit card companies thought I was a good investment; they saw I was a hard worker, had good grades, thought I would earn raises soon. Of course, should the "credit card company" have other investment opportunities, I could invest the ten thousand dollars with them. Or I could invest somewhere else. The web would grow as I invested money, and income from those enterprises flowed through me back to the credit card company and then back to its investors.