I think there will still be borrowing and this won't go away. However the interest rate with a money that inflates less than 0.1% annually in 21 years cannot be as high as it is for a fiat inflating 7%+ per year on average. Given that bank deposits are not able to sustainably yield 5% in fiat now, I don't see how can Saylor expect BTC deposits to sustainably return 5% in interest in 21 years. Maths simply don't work, that BTC denominated interest rate will have to come down.