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).