Thanks for IMO. You wrote "You seem resistant to other peoples' opinions on the matter - so good luck with your chosen AA and hope your crystal ball works better this time." I am open to other ideas, but 58, single, have to take care of myself. Worried about losing 20-30% or more; then back to work at 70? P.S. I'm relying on other investment advisors advise: Prechter, Weiss, Stack, Shiller. I'm just not that savy.
Just my opinion, but you are too focused on the infrequent and short-term. To wit, the market has declined by more than 20% just twice since 1975, and has hit double figure declines just four times. Meanwhile, the average return has been right around +7.0%. The largest individual decline in the market since 1975 occurred recently in 2008-9, but the market recovered its entire value in less than three years.
You cited deflation happening many times since 1800:
Deflation has occurred 54 times since 1800 (26% of the time). Of those, 40 occurred before the year 1900. Prior to 1851, the "CPI" was based on prices paid by Vermont farmers to support their family. CPI didn't even come into existence until 1890.
Since 1890, the "more accurate" (my term) CPI has indicated deflation only 16 times, and only
once since the 1950s (in 2009). Conversely, the average
inflation over that same period is approximately 2.5%.
If I were you, I would plan for something that is most likely to happen, otherwise you become the financial equivalent of a "prepper" (someone who invests in canned goods and ammunition in the event of the zombie apocalypse). Risk does not just mean "losing principle." Risk also means "losing purchasing power" which can happen either by loss of principle or via the inevitiability of inflation.
The key is to find a plan that can weather tough times in the market, but also ensure you come out ahead of (or at least equal to) the effects of inflation. You sound to me like someone for whom a 60/40 stock/bond allocation would work well. Moderate risk with good potential for growth above inflation. I also suggest the Bogleheads wiki as a good place to start where no one's trying to sell you anything.
Bottom line: inflation is
highly likely to erode your portfolio (and thus your standard of living if all-cash) over the course of the next 40 years. While an equity downturn WILL happen again in your lifetime, its effects will likely be short-term, and a properly allocated portfolio will allow you to get through the downturn without going back to work.