What is the definition of fair value? The fair value can be decided by just 2 things, the price people willing to trade, and the mining cost of one Bitcoin.
It is a complex concept and it is even more complex in the case of bitcoin. When people try to establish the fair value of a companys shares, they take into account their total assets, total debt, cash flow, past recent and projected growth etc.
In the case of bitcoin, you dont have any future expected cash flow from it, nor debt or assets. Guessing a fair value consists in guessing adoption and mining costs, as you say.
Talking about the fair value of some shares has more sense than talking about the fair value of bitcoin, although in both cases it consist in guessing but in the case of shares it is backed by more numbers.