I've been invested largely in US index funds for a while now, and I've definitely thought about this. My conclusion is S&P 500 is too big too fail, everyone with various forms of power in the US (economic, political, etc) are incentivized to keep the music going. Sure it feels unsustainable, but there is no way going active can help me—I don't have enough access to the right people, and even if I did, it's better as a hedge strategy. Someone who has a billion dollars can easily pace a bunch of $10M bets on long-shot hedges that will mint a fortune when the music stops. Theoretically I could do something similar at smaller scale, but the people smart enough to have credible strategies are not talking to me, and even if they did I don't have the expertise to judge the advice (the super rich don't either, but at least AUM volume is some signal of competence).
The S&P 500 can and will crash at some point. It has and it will. That's part of the lifecycle of market psychology. We go through cycles of over valuation and under valuation. It's true there are many forces with interest in keeping the markets up but there have always been and it's always crashed because once the psychology changes there is no amount of intervention that can keep the market up.
If you are invested for the long term then just don't think about it. If you want to diversify a little then go for it - slowly. Also keep in mind your US index fund is full of international companies anyways.
I'm along the same lines of thinking. I got most of my funds in SGOV (I manually did t-bills for a while but too lazy to keep it going).
> there is no way going active can help me—I don't have enough access to the right people
It really comes down to this realization. Without access to what all these billionaires and their companies have access to, I just feel like a pawn with everything to lose.
