Looks like its time for each of us ask the question 'If a downturn arrives and lasts several years can I afford to hold, in hope of sharing the rare extraordinary days that account for most of the long term increase in asset prices?'. Holding long term is what distinguishes above-average returns but not everyone can afford to risk shorter term losses.
Personally we can afford this gamble, our standard of living is mostly covered by SS and good pensions so the better-than-expected investments at this point will eventually go to the kids. Not too soon, we hope.
I think we'll hold. This strategy served us well in 1987 (22% market drop over one weekend, a couple of days later I put 50% more into S&P index, a few years later I was able to retire years sooner than I had initially planned), Y2k, 2008, etc. We hope to also share in future upturns. Basically we're betting on America overall. Alternately if things do go all to h- we can still afford to buy some income-producing assets that are then available at fire-sale prices.
Buy and hold is a worthwhile strategy.