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 )
The argument needs a very simple logic. When you fork Bitcoin, the fork is not Bitcoin. It simply means you are releasing another coin from Bitcoin which can never be called Bitcoin anymore. Do you think you can have a forked Bitcoin which is also called Bitcoin? No. In this sense, Bitcoin Cash, Bitcoin Gold, Bitcoin Diamond, and so on and so forth are not Bitcoin. They may be forks of Bitcoin but they are essentially alternate cryptocurrencies.