Upon retirement place half of your retirement funds in Berkshire and half in a Nasdaq 100 equal-weight ETF, such as QQQE.
...
That is still too much concentrated. In tech.
Problem is, people want to get the returns of a concentrated portfolio without the associated volatility.

Well, yes and no. Something like QQQE is going to be price volatile, but volatility isn't risk, most especially in this context. The risk is very low, much lower than SPY or QQQ or most equity funds, since no position is over 1% of the assets. Company specific risk is extremely constrained. If the AI bubble were to burst badly and a half dozen firms went bankrupt, it would still only be 6% of the money. If you're in a cap weight fund, it's "which six?" becomes an existential question.

Admittedly there is the problem of "everything in a bubble" as in 1999. Or "everything went bankrupt", I suppose. But, little noticed at the time, real average earnings among the Nasdaq 100 set reverted [pretty much] right back to the prior trend after the 2000-2002 recession and bear market, aided a bit by periodic index reconstitution. On a larger time scale value just kept chugging along, with temporary dips in recessions that fully unwound after.

So, if your concerns are high returns and smooth returns and safe returns, once you realize that smooth isn't as important as you thought, you can still get high and safe.

Admittedly, a BIT of smoothness is good. I wouldn't want to have retired in 1999 owning nothing but a diversified basket Nasdaq 100 firms, even knowing that the value was still chugging along. Assuming one is over a certain age, arguably the best way to increase smoothness is just to put a certain portion of your money into an annuity. The older you get, the more sense this makes.

Jim