There is a counter argument for the 'diversify your portfolio' ideia. Why would you diversify, if your betting on the right winner. For example, you shouldn't buy any alt coin that gives you less % result vs ETH. You can go even further, you definitelly shouldn't invest in anything that does less % than bitcoin.
But there's this argument that it's usually to reduce the risk of failure and to bet on the market as a whole
Let's say you buy Tesla, you're not betting on a good economy, you're betting on Tesla. But take an index fund then you're betting on the economy
Same way I treat crypto, I can't trust just one project to be the future of crypto, but I sure do bet on crypto
in terms of risks, everything after BTC and ETH is much more riskier
so just buy BTC/ETH
but we can clearly see that some good alt coin projects give you really good returns.