"and I say—stop."

I take it you're not a fan of tactical asset allocation. :) I'm not either, but even Buffett sells when the price is too far above fair value, as he did with AAPL. I considered trimming a little BRK.B when the P/B was 1.78 on May 2nd, but I didn't. I would have trimmed if the P/B had been 2.0. Too bad I didn't trim, though. Trimming would have saved me a 10% loss on the shares that I would have sold. And if BRK.B had risen 10% from May 2nd instead of fallen 10%, I would have been OK with that. I just have to buy, sell or trim when I think the expected return indicates doing so. It's too late to trim now, imo, as I think that BRK is selling at about fair value, not 15% above fair value. When the expected return on BRK.B or SPY or whatever is less than the expected return on T-Bills, it's time to at least think about trimming. I would definitely trim SPY, or VOO, now. Now is not the time to be heavily invested in the S&P 500. So much for 90% S&P 500/10% short term treasuries. Even though over the long term the S&P 500 will outperform short term treasuries, almost anyone will do better holding less SPY right now.