It may be insane and retarded for it to exist, but that doesn’t mean you can’t benefit from it. And apparently a 50 year mortgage is sinful usury, but a 30 year isn’t? So where’s the line then?

30 year is too. The line is at fiat banking and artificial debt contracts. Lending money needs to have real risk associated with it so we can price capital correctly. That's one of the main problems Bitcoin is trying to solve. Nobody is actually lending the money, the contracts are fake, the money is fake, and the interest rates are fake. That's where I draw the line.