I get that there is only a finite (21 Million) supply of BTC.
But, The problem with cryptocurrencies generally
is that there's potentially an infinite amount in totality.
I mean even bitcoin is now forking and sort of spawning brother and sister coins like bitcoin cash.
What good is it if you can spawn a new one every day?"
So why should any Crypto (including BTC) be worth anything ?
This is such a complex question with such a simple answer...
I'll start by saying that just because the bitcoin blockchain forks doesn't mean that it's bitcoin. Bitcoin Cash and Bitcoin Core are the two major competing "Bitcoins", but as far as the mass majority of users are concerned, Bitcoin Core is still considered
THE Bitcoin.
Regarding the alternative cryptocurrencies... Essentially this doesn't mean anything when compared to Bitcoin's finite supply. It's almost like how the prices or availability of silver, copper, bronze, steel, tungsten, etc rarely affect the price of gold. Yes, there is slight influence, but not enough to truly impact the longevity of the value of gold.
Coins that are newly "spawned" generally fall to the wayside. I would say that over 80% of newly launched altcoins are rendered ineffective within the first 100-day lifespan of the coin, and the remaining coins exponentially kill themselves off within the first year.
Bitcoin is like the "gold standard" of the crypto world. Other coins use blockchain technology to try to create a coin, but it will never be Bitcoin. It's as if you were to create a new fiat currency and say that your new currency is now worth the same as the US Dollar... No one will value your currency just because it's new. And chances are that your new currency will never reach the market cap that the dollar has reached. The same principal applies with newly created coins. Unless they're revolutionary and adopted by a mass following of users (such as the top 10 on coinmarketcap; hell even the top 20), these coins will always be worth very little compared to the big dogs.
The major cryptos will prevail while the shitcoins fall out of the cryptoeconomy. And Bitcoin will more than likely continue to remain the gold standard of crypto, which will allow it to not only retain its value, but continue to grow its value.
Hope this kind of answers your question.