Excellent point. The coinmarketcap.com site lists 3882 cryptocurrencies. That makes the "Bitcoin's limited supply means permanent value" talking point ridiculous. They list 91 things with "Bitcoin" in the title, 6 of which are in the top 100 by monetary supply, and all 6 notionally have at least $100m in circulation.
So long as everybody knows what the "real" Bitcoin is, it doesn't present a problem, anymore so than existence of the Liberian dollar renders USD unusable.
Not at all. From a market perspective, Bitcoin is not scarce in the same sense that gold or diamonds are scarce. If people want to buy into cryptocurrency, Bitcoin is far from their only option. As this whole discussion around Ethereum makes clear, other cryptocurrencies are viable. As I mentioned earlier, other Bitcoins are clearly viable.
Yes, the number of Bitcoins in a given fork has a limit, assuming whoever controls that for keeps it that way. But that doesn't matter much if market demand is such that other forks or entirely new coins become viable.
it's like saying gold isn't scarce because there are other metal compounds. you're not making sense at all. if you want to interpret scarcity this way - feel free, but be informed that pretty much everyone else will look at you weird.