Here's a relevant example that timing isn't that important in the long term:
https://www.cnbc.com/2015/08/27/the-inspiring-story-of-the-w...
It also only analyzes a single time period (1970s - 2015), which happens to be quite a good one for stocks, and then makes an implication about markets in general which is quite speculative.
https://fredblog.stlouisfed.org/2019/12/how-has-the-u-k-stoc...
Notably ~1917 is when global power shifted from the U.K. to the U.S., cemented in 1945.
(not sure if they included dividends - would probably change the calculations, but for sure you see that 1815-1900 was a ~85 year general bull market in U.K. stocks, followed by ~75 years of real-term declines from 1900-1975).
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Also, the U.K. stock market went nowhere from 1874 to 1952 in pound terms, a whopping ~70 years!