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.
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.
Diversification really helps when you know that the price will be going down, but how you can predict them if the price is unpredictable.