Isn't this the quintessential explanation of why deflationary environments are so bad for an economy?
deflation -> depressed borrowing -> depressed construction -> job loss -> more bankruptcy / defaults on debt -> deflation
Well, one way of looking at it, it's all just gambling, this just reverses the way it goes.
When you take out a mortgage on a home, you're gambling that the value of your home will increase over time ahead of inflation. The bank is gambling that the interest and fees they get from you will earn them more money than inflation eats into the principle etc.
Perhaps a Bitcoin mortgage has some sort of fixed reduction in the payments/interest over time that accounts for expected deflation. Or maybe they just front-load the principle, unlike the traditional mortgage, or other financing like a car loan that front-loads the interest.
(don't even know if that makes sense, just a random idea pulled from thin air...)