From your statement it seems like you're underestimating it's impact.
Their impact would be felt across the whole market by their sheer valuation - even if you tried to exclude them specifically.
So yeah, if eg ai crashed and took Nvidia, meta, goog or MS for the ride... You'd have massive impacts all across the board, even if you specifically tried to exclude them from your index, just because of how gigantic it's share on the economy is.
But this is purely theoretical. It can only be considered an opinion until something actually happens - because the market has never been in a situation like this before - no matter what some people may claim.
If you have a $10B fund which owns things evenly across the nasdaq, and a new company arrives, you have to buy shares in the new company, which means selling existing shares or getting more funding to balance.
So you buy $1B of SpaceX stock at the IPO price, meaning you have to sell 10% or $1B of existing Nasdaq stock, which (when combined with every other fund) lowers the cost of the other stocks.
If I own stock in everything-but-spacex, then I'm seeing the price collapse because everyone else is selling.
Then if spacex stock collapses to half its initial price, your $1B becomes worth $500m, you get margin called, and you have to sell more stock, pushing the other stocks down, and the problem cascades
Of course the recession that would likely occur if/when the AI spending merry go round comes to an ends would have a negative impact on the entire economy. In that scenario, would you rather own a stock that's going to zero, or one has taken a hit because of the other company going to zero but still has a viable long term business?
Would you rather have an unusually large concentration in a few very specific companies in your portfolio, or have increased diversity (and therefore lower risk) in this unprecedented scenario?
