Berkshire went heavily down not in the era of the bursting of the dotcom bubble.
It went down in the era before, the dotcom hype era.

I think the composition of Berkshire has changed since 2000. With all the non-insurance businesses it added since then, Berkshire pretty much reflects the overall US economy. I suspect Berkshire won't be spared this time around (unlike the bursting of the dot com bubble, and similar to the GFC) if there is a major correction in the S&P 500 index. But the correction will provide Greg the opportunity to put the excess cash to work. I also think the correlation of Berkshire to the index has increased significantly in the last 20 years or so; index/ETF addition, higher business correlation to overall economy, and increased passive indexing are likely culprits.

One note on the excess cash: it's not $360B as some state. It's more like half of that. About $180B of that cash will never be invested in stocks or whole businesses as it belongs to insurance policy holders & thererefore will be invested very, very conservatively in short term T-bills.