I wouldn't be too sure about such stuff. If for some reasons, for example, the USA invent some strong laws against cryptocurrency, they'll drop hard. Really hard. Something like that could lead to the end of some coins. 3-4 coins are not well enough diversified inside cryptocurrencies and buying only cryptocurrencies is no great idea at all imo. I wrote an essay about Bitcoin three years ago and many of the problems I found to be the most important are still there. Bitcoin is just an example, but most of the cryptocurrencies have some serious weaknesses imo.