Well the point of index funds or index ETFs is exactly that... just copy an index.
And in a recession / depression, it is probably bad idea to "copy" the performance of S&P500 or MSCI World. So I definitely agree that it is a dumb idea to "put it all into VTSAX" and similar strategies that a re promoted on various "FIRE"blogs, on several subreddits and many blogs.
ETFs by themselves are not a bad tool or bad idea but of course you must have a goal and you must know what you are doing and why. For example - do you predict financial problems of US economy and you see some undervalued economies such as Russia or Eastern Europe? Then buy ussian or eastern european ETF. Do you think energy or oil pipelines are now undervalued? Buy and energy or oil ETF...