First off, if the 5 year treasury rate ever approaches 1000%, I assure you the Fed will collapse.
Second, problems with hyperdeflation and hyperinflation are both rooted in the hyper.
If you can buy a pizza today, or have $10 million dollars in a few years, why buy a pizza?
Third, the issue isn't just deflation, it's specifically deflation of a currency. If you invest $100 in a company, there's a chance it will be $1000 next year. But your $100 will circulate in the meantime and generate value.
Now if instead there was just as good a chance you could put $100 under your mattress, and next year it'd magically have become $1000 despite never having left your grasp... why would anyone ever invest again? You get to keep your money and have it increase in value, there's no incentive to invest or spend any more than you absolutely have to.
But then as people spend less and less, there's less and less economic activity being generated. You stop wanting to pay people so jobs disappear. Eventually you end up in a game of chicken where everyone is scared to spend money.
A BTC based country (fully BTC based, not El Salvador's lip service) would be an utter hellscape devoid of any economic output other than people waiting for their currency to appreciate. Not unlike the real thing...