There are plenty of great ETFs. If you had a 20 year time frame, putting it all in a SP500 ETF is certainly not a bad option (though I would personally diversify to maybe 2-4 ETFs to get international and fixed-income exposure).

edit: See https://www.bogleheads.org/wiki/Three-fund_portfolio for an example of a simple, well diversified three ETF portfolio.

Not a comment about ETFs, but in general, the word "foolish" applies to someone entering the scheme, not someone running the scheme. And as is the way of humans, there are always enough people willing to be misled into bad decisions. In that sense, these schemes are very smart.

To repeat, this is NOT a comment about ETFs; it's only about where the word "foolish" goes.

Never put all your money in any one instrument even if it is a collection of many different things. The foolish part was 'put it in a single ETF'.

Honestly this whole thread is a dumpster fire I'm leaving this one alone.