one of the main purposes of a portfolio is to diversify into stand alone assets that could save you in case another asset went down. but when bitcoin goes down all the altcoins go down, in most cases they go down so much bigger than bitcoin. that means it makes no sense to buy them and then call it a "portfolio".
That's a good point.
Buying a lot of cryptocurrency doesn't mean you are diversifying, as you are still exposed to the same risks.
One Elon Musk tweet about PoW energy consumption is capable to crash the whole market, even PoS coins (which doesn't expend energy) are affected the same way
Look at CMC 7days charts. They are basically all the same:
So you are buying altcoins not to diversify risks... but to try to get more money than a full BTC portfolio.
Personally, I have more than 80% Bitcoin and 3 more altcoins, which I believe they have good fundamentals. They are all top15 and have somewhat established position in the cmc and large capitalization.
I think buying small cap coins is too risky, but it might be interesting. I recommend to anyone who asks for my opinion to buy at least 50% bitcoin in your cryptocurrency portfolio.