The argument is again and again that the value of Bitcoin will increase because supply is limited to 21 million. But this is quite simply wrong. Every time Bitcoin is forked that is in real terms an expansion of the Bitcoin currency beyond 21 million. To some degree the same is true of alternative crypto currencies which will flood the market. There is already more than 1000 cryptocurrencues (
https://en.wikipedia.org/wiki/List_of_cryptocurrencies )
I think it is more limited how we are think. I think there are no 21 millions BTC on the market. At least %20 may be loose already. Est. 17 million BTC in the market for today. But somehow, someone has already lose their key or they've dead (sadly) and no one knows their private key. For example, my friends has lost his private key and he can't reach his wallet (0.5 BTC)