If the only thing you could buy were SPY, you'd be right; I'd sit on
cash and wait for a great entry point.

This is unfortunately a major issue facing the vast majority of passive investors who intelligently choose to index rather than to pick stocks without investing the required time and work. Obviously, these people could choose an active strategy and delegate management to someone with a proven track record ... however, it is also not easy to pick a successful manager.

Would it be insane for a passive investor to allocate a large percentage of US equity exposure to Berkshire? In some ways, it might be if the individual doesn't have sufficient personal understanding of Berkshire to avoid panic in a bear market and is unwilling to learn about the business. But for someone who wants a relatively passive strategy and is willing to invest maybe 10-20 hours per year following Berkshire, this could be a superior approach. I really don't think it takes much more than 10-20 hours per year to keep up with Berkshire at a level sufficient to be intellectually armed against panic selling during a bear market.