The Mid cap SP400 has beaten BRK-A for 20+ years ( since 1996 ) and most rolling starting points in between . In light of this, when Buffet says that investors "would do well with just investing in an index fund", he's been right. Accordingly, as one of the one of the golden rules of investing is to "not take "outsized" bets", this low odds endeavour seems to have caught up with BRK's performance stats ...
1) Buffett aims to beat the S&P500, not the mid cap S&P 400 or the nasdaq 100 or the nikkei or the ftse100.
2) The golden rule of investing IS to take outsized bets when warranted. Probably the only way to beat the averages.