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 heard that argument over and over again, but I don't think it is a valid one. When you are forking bitcoin, you are not altering it's supply. You are creating a new coin, that needs to have a set of different rules, and so it cannot be considered bitcoin, so you can't say it's adding more coins to the BTC max supply.
The altcoin that results from that fork, could be accepted or not, and even if they are, they will not be confused with BTC.
Are you saying that bcash, or bgold is bitcoin? Are you saying these coins are an extension of bitcoin? They are not, so you can't say BTC limit supply was changed. You saw what happen after the B2x failed fork. Bcash was being pumped like crazy, and BTC price was decreasing. So basically these coins were "competing" with each other. Given this behaviour how can you say that the bcash fork was an expansion of bitcoin? They are completely different, and no mistakes should be made about that.