If we put aside additional features and base ourselves on the core blockchain protocol, then Bitcoin and Ethereum may be the only cryptocurrencies humanity needs to participate in the decentralized economy.
I tend to disagree with that, especially when I think of how many times I've tried to use bitcoin as a currency--or even just to send it to another wallet--when the network is extremely congested and it takes an abnormally high fee just to get the first confirmation. If bitcoin was the only crypto in existence, that just wouldn't be good IMO. And ETH is used much less as a currency and more for its smart contract functionality (as far as I know).
But I do think the crypto market is extremely saturated, and I've thought that for a long time. Even coins that would function much better as currency than bitcoin (Dash, doge, LTC) don't even get used in that capacity. They're used as trading instruments and/or stores of value, and seeing as how there are over 6500(!) cryptos on the market, I don't think there's nearly enough interest or money to support most of them.
I'm assuming that 6588 figure includes coins with dead blockchains, like the shitcoins you'll find in the 1-satoshi graveyard on Yobit. That's where the majority of them are headed IMO, since most don't offer any advantage to anything else on the market and most are just cash grabs by "developers" who copied the code from some other coin.