How much does the US stock market have to fall to make up for all those decades of Berkshire's idle cash?

Opportunity reserves are completely useless.

Since 2009, Berkshire's T-Bills have yielded less than 1% per year — well below inflation. Meanwhile, the S&P 500 has multiplied its invested capital by almost 6 times in the same period.

It doesn't take much math to see that holding so much cash was a disastrous decision.