Stable in relation to what? People often say 1 BTC = 1 BTC. If we go by that logic and say $1 = $1, we have to discuss the purchasing power of that unit or asset. Think about what you could have bought with 1 BTC 10 years ago and what you can buy today. Now consider the value of the USD 10 years ago and what it is today. Go back even more like 40 or 50 years into the past to see what the money printer has done.