How Bitcoin levels of speculation desirable for a currency? If my national currency was going up like bitcoin spending would decrease a lot. Why you'd you buy

How Bitcoin levels of speculation desirable for a currency? If my national currency was going up like bitcoin spending would decrease a lot. Why you'd you buy a car today when you can buy it in a month and spend 20% less? The economy would stop functioning. And when the value of currency falls people want to spend it as fast as possible.