People will be reluctant to spend a bitcoin if tomorrow it will be worth more. Yet, this is how bitcoins ALWAYS will be.
People claim this a lot, but it's dead wrong.
The only five reasons BTC goes up in value from a fundamental perspective are:
1. It becomes more useful
2. Expectation that it will become more useful
3. It gains some kind of non-utilitarian subjective value, such as being a prestigious collectable.
4. Fiat goes down in value so the
exchange rate for BTC goes up (not really increasing in value but appears to be).
5. It becomes more expensive to replace (i.e. difficulty goes up, electricity goes up, etc.) I don't really agree with the argument that difficulty doesn't influence price (only the reverse) and I believe I could show that formally, but I don't have time to write it down.
Just because there is a fixed or declining supply of something doesn't make it go up in value. There is a fixed or declining supply of one year old dog shit, but no one is going to pay more for it, ever.