It was not designed to be fully repaid, it was designed to create bigger revenue so we could have bigger debt. If you had 100 income, and 150 debt, then you used that 150 debt to make 150 income, and then get 250 debt, and keep repeating that until you make 2500 revenue and 4000 debt, and it goes on forever.
This means the debt is not designed to be paid, it was never meant to be, the new revenue pays the old debt, and new debt paves the way for bigger revenue and so on and so on.
It's basically like the debt is the core of modern economic system and is meant to exist from beginning until the end.
The reason why developed countries are maxing their GDP to debt ratio is because they know damn well this is how it works and they want to take advantage of it well. Country like japan, is becoming rich because they know how to play the game.