When Bitcoin first hit public consciousness the knock from economists was that it had a built-in deflationary spiral and that seems to be true. The price keeps going up and up with a few noted bumps. Rising value is great for speculators but it's a death knell for an actual spending currency. You'd be nuts to spend it if you expect it to appreciate. That's why central banks aim for low but positive inflation.
I think the difference here is that Bitcoin is predictable deflationary vs fiat being unpredictable. If you can know in advance the rate, it becomes sorta like an investment vehicle, where instead of dividends you get appreciation of the assets.
To look at it another way - why one would spend $100 from their brokerage account if they know a year later they can spend $110?
Bitcoin is not remotely predictable. The value has swung wildly over the past 5 years. Dropping more than 50% then gaining 200%. By comparison, USD has been rock solid even with the recent run of inflation. An actually circulating currency causes a panic at an 8% drop in value and yet there was zero macroeconomic impact from BTC dropping 50%. BTC is less stable than Turkish Lira.
The same can be said for stocks, and they are considered a good investment if you are in the knows. As an example, Tesla lost a third of its value this year.
This is a fallacy.
It's rational to buy stuff when it's cheap and sell stuff when it's expensive. Not the other way around. In other words, if the money gets suddenly more valuable, people would go on a shopping spree, which would cause inflation and the value would return back to normal. They wouldn't wait for it to be even more valuable. The situation would be different if there was a guarantee of deflation, but there isn't.
Economic demand is driven by human needs and wants. Lot of everyday consumption, like food, is bought when it's needed, and can't be bought 5 years later, because you'd be dead already. Other things, like a computer or a car you can buy later, but then you'll have to make a calculation whether the thing is more valuable today or after 5 years if you'll get it 30% cheaper. There's no situation where you sit on a pile of bitcoins and just die there waiting.