Scarcity alone is nowhere near enough to make something valuable
Apart from being scarce, to be valuable a thing should be actually useful at that. So the fact itself that there are only 21M bitcoins and over 7 billion people in the world doesn't tell us anything in this regard. For example, once there was a coin plainly dubbed 42Coin. 42 in the name referred to the amount of total coins to be mined, i.e. just 42 coins. It was expected that one coin would cost around 1 million dollars since when it started Bitcoin was around 1,000 dollars per coin which would justify that price if it proved to be as useful as Bitcoin. Obviously, it wasn't and thus it quickly turned into crap worth of nothing. So it kinda looks it is you who should first think about it