The thing is that if we did not have the concept of money then I believe your argument is valid. However since we do, the very expectation/chance that Bitcoin could become money is what I guess drove people to want to have it in the first place. In other words: Once the concept of a medium exchange for trade is established you can evaluate any good with regards to how well it fits to the „ideal“ medium of exchange with regards to its properties. Because of the absolute scarcity of Bitcoin (and some other properties) it becomes valuable and people want to have it