I think this is a fundamental issue for Bitcoin as a currency. If it were to increase in value by a significant amount on a regular basis — even "just" 10% per year — why would anyone ever spend it?

The USD decreases in value by 2-3% each year, so people have incentive to spend dollars rather than hang on to them. But an appreciating currency creates the opposite incentive. So why would people ever use Bitcoin as a currency?

Edit: To clarify, I'm asking why people wouldn't just spend their USD — a currency that is going down in value — instead of their BTC.