The cryptocurrency market without Altcoins will lose investment attractiveness. There should be a large number of different coins on the market.
How it can lose attractiveness if it's just bitcoin on the list?
Before the craze of cryptocurrencies, bitcoin has been known for it and the idea came out from others and that's when the altcoins were born. Whether there will be alts or none, the crypto market has huge potential even if it's bitcoin alone. Think of how much bitcoin would be if the total market cap is on it.