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 with imperfect knowledge it's a solid choice for nearly every situation. Having it as a default choice (that you possibly move away from after doing research) is much better than not having it as a default choice.
with imperfect knowledge it's a solid choice for nearly every situation
I see no reason to believe so. Do you have evidence or arguments to support this naked assertion? What other choices are you comparing it against?
P/S/A: There are single sentences which can create life-changing amounts of difference.