alt Hacker NewsMarket neutral index fund investment can not be accurately described as fueling a bubble, though. You're neither bullish nor bearish on Mag7 in particular.
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With massive concentration like this you are forced to be bullish, as your success is predicated on the bubble not bursting.
How does your first premise follow from the second? Yes, you're not bullish on these companies specifically, but you're effectively giving them (and some others) your money and inflating their valuations. You're bullish on the status quo and they're a large part of the status quo.
In fact, it's arguably worse than deliberately investing in them, because you're completely indifferent to the soundness of their business model. The hope is just that through the magic of "efficient markets", someone else will arbitrage your errors away. I don't know if there are enough takers for a bet of this size.
You are copying what active investors (in aggregate) are doing so if they’re fueling a bubble so are you.