Loan contracts, the capital of the banking system and collaterals of the borrowers tell me that I'll have that 90% in 10 years.
Nope. It may tell you the amount of dollars you'll have, but not their future purchasing power.
If only we knew such thing...

Purchase power is just a concept. Useless concept. You cannot compare dollars from 1930s with the one that are in circulation today. The loans from 1930s have long been paid off. Today's dollars are maybe from the luan that was issued last week. So you are comparing apples and oranges.