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 should allocate good percentage with Bitcoin since this can be your safe haven while the altcoins are not working properly, holding Bitcoin is always advisable same thing on diversification. Never go all in even on Bitcoin, that’s still risky and not a good way to invest. I do hold Bitcoin and trade altcoin as well, this is very effective for me.