Can't comment on that, but for stocks, definitely yes. Historically, the market has always recovered from slumps. I know this does not mean it always will, but lots of people dropped out of the stock market after the crashes in 2001 or 2008 - for them, it would have been wiser to hodl.
Historically, this isn't even a slump (yet). The dow can drop another 3,000 points and still be up 1,000 points from what it was this time last year.
What's the expected recovery time? (Say average for a randomly selected 10 indexed stocks).
FWIW this is an academic enquiry, I'm too poor to gamble.
I'd find it instructive to know the answers. I can't imagine no indexed stock has ever failed, nor that the indexes are a perfect insulation - you wouldn't need FTSE250 if the 100 was perfect, they do moderately different things.
Indexes are picked by "stock pickers" they're just picking conservative long, wide spreads AFAIK.
Yes, no index is perfect. But most of them provide more diversification than the average person would be able to achieve by manually managing a portfolio.