"The problem is, our largest acquisition to date, PCP was an unmitigated disaster, as was the recent Pilot acquisition, the Lubrizol purchase and the Kraft/Heinz debacle."

You left out PacifiCorp.

"...buy huge into the S and P 500 Index or individual names when valuations become favorable.
Explain why this would NOT be preferable to buying 100% of private businesses?"

Well, for one thing Berkshire can move profits of wholly owned subs elsewhere.

"the track record has been meh, hasn't it?"

I'd like to see the track record, both for 100% acquisitions and for equities. Pointing out some losers, in acquisitions or equities, doesn't give a quantitative picture. I take your point, though. The track record for corporate acquisitions, including Berkshire, isn't exactly glowing. Overall, though, I suspect Berkshire's track record for acquisitions is not bad. We need numbers.