Buying only Bitcoin or at least 90% Bitcoin.
I do agree that BTC must be your primary target of investment in crypto. IMO 60 to 70% of your capital should go in BTC and rest of capital for strong alts (top 10 alts at CMC) and short term trading. OP point is also valid that when BTC goes down every coin goes down with it. We have to accept that fact, the logic of equity trading don't fit here.
I would rather agree if we put 100% of our investment was on bitcoin and just fully ignore altcoin because they are completely useless without bitcoin.
You will diversify your portfolio if you are not in crypto and stick to bitcoin investment only if you invested in stocks and real estate but don't have profit yet, I would go in bitcoin. Last month, some of my friends diversified there investment from stock to bitcoin because they realized that bitcoin has the potential to gain more profit.
Well, it's sometimes to consider that some altcoins are more profitable than Bitcoin and this is found to be a reason why people don't give the 100% investment in Bitcoin alone but they still spare some funds on altcoins. A matter of fact is that we can see huge demand on altcoins which probably it means that more investors are investing in altcoins and diversifying their portfolios.
If we just look for long-term investment, Bitcoin is the best choice but seems it was not the best deal for now. And for me, I use to spread my funds to altcoins as well while leaving 50% on Bitcoin for more profit assurance.