You're not modelling the contagion here. The problem is that while any single one isn't that big a share of the S&P 500, similar companies do make up a lot collectively. Excl some non-tech/AI firms:
NVIDIA Corp NVDA 8.02%
Apple Inc AAPL 6.53%
Microsoft Corp MSFT 4.84%
Amazon.com Inc AMZN 4.01%
Broadcom Inc AVGO 3.36%
Alphabet Inc GOOGL 3.32%
Alphabet Inc GOOG 3.09%
Meta Platforms Inc META 2.23%
Micron Technology Inc MU 1.71%
Advanced Micro Devices Inc AMD 1.19%
Oracle Corp ORCL 0.99%
That's 40% of the S&P 500.
And if anything happens to the AI bubble all of these go down together. While they won't all go to zero and cause a "-40%" overnight, Nvidia's rise is so meteoric that they will trigger a -8% and the rest's valuation has more than doubled since 2023. Even Apple, which isn't much of an "AI company", is still following the AI-tech hype.
If Nvidia eats shit, and the others go -50%, that translates to an overall ~-24% on the stock market.
Before any contagion outside the tech industry is considered. Look at the Dotcom Bubble and a -40% to -50% crash is quite plausible.