Most of the personal-finance-advice industry is parasitic and/or self-deluded
True.
...and it's generally agreed on by economic theory and experimental measurement that an index fund will deliver the best returns you can get without huge amounts of effort.
Not true. Or at least much more complicated.
The first problem (minor) is with the concept of an "index fund". Most people read it as an S&P500 index fund, that is, a fund that passively invests into US large-cap equity. At this point it's reasonable to ask, why US large-cap equity? Why not a US equity + US bonds passive fund? Why not add European stocks or bonds? Why not add emerging markets? Why not add commodities or real estate? The answer to what kind of a mix of asset classes do you want to hold is... complicated.
The second problem (major) is with the concept of "best return". It's pretty meaningless to talk about best returns without involving uncertainty, volatility, and individual risk tolerance. I don't think asserting that the risk-return profile of the S&P500 (again, US large cap equity) is just perfect for everyone is a defensible position.
Finance is a hard problem. There's no easy universal answer to "what should I do with my money" -- the answer is it depends. Sure, compared to investing in S&P500 there are a lot of worse things you can do -- but that doesn't mean buying the Vanguard index fund is the optimal choice.
Finance is a hard problem. There's no easy universal answer to "what should I do with my money" -- the answer is it depends. Sure, compared to investing in S&P500 there are a lot of worse things you can do -- but that doesn't mean buying the Vanguard index fund is the optimal choice.
The argument is that the added value of the "it depends" answer doesn't pay for the additional cost in calculating what it depends on. With perfect knowledge, not everyone would go for the Vanguard index fund, but that's not the situation we have, and...
P/S/A: There are single sentences which can create life-changing amounts of difference.