A portfolio of 30% BRK and 10% each of the Magnificent 7 may work out well...

That's an open question.

It certainly would have worked out well in the recent past to hold only the very largest firms. Since 2015, say. (other than the 2.5 years ending Dec 2022)
But it would have been a guaranteed way to underperform the cap weight index in the prior many years. Statistically the very largest firms have been terrible picks through history...till recently.
Random example: an equally weighted portfolio of the 8 biggest US-domiciled firms would underperformed the S&P 500 by 2.7%/year in the 19 years to end 2015.

So, you have to decide whether the outperformance of the biggest giants is the new rule, or just a passing cycle.
I definitely don't know the answer, but I would be very hesitant to build an investment strategy around the assumption one way or the other.
Especially right after the giants have been on a tear.

Jim

Berkshire Hathaway FAQ

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