This is why otc tradfi crypto futures are needed. You can buy a train car full of corn a year out trivially on a liquid futures market, regardless if corn prices fluctuate wildly you will pay what was agreed. Listing BTC derivatives would solve the predictability for fixed loan terms.

Fiat currencies also can and have lost half their value plenty of times.

Actually I can't think of any fiat currency that hasn't at least halved in value?

Usually over a long time. They are a lot less volatile than Bitcoin.

You can also offset the risks of sudden devaluation through matching assets and liabilities, trading derivatives and so on.