Cryptocurrencies are too much correlated to Bitcoin. If he plunges then all altcoins will follow the same path or even worse. For me, buying only Bitcoin is the best thing to do at this moment.
On the link below you have some plotted data:
https://cointimes.com.br/investment-diversification-in-cryptos-bad-idea/For example:
Correlation between Ethereum and Bitcoin is 0.82 which means a very strong correlation. In 2017 the same correlation was 0.41. For me, if you're diversifying in two types of assets that have the same correlation you're doing it wrong.
What do you think about it?
Diversification is not bad at all. Actually it's a matter of choice if you do diversify you need to make sure that your choosen crypto has not correlation to each other therefore you can't be wrong in doing diversification. There some other cryptocurrencies that correlation to bitcoin that can be used for diversification. However, Bitcoin is the best investment at all.