Since 1900, there have been two sets of conditions in which it has been extraordinarily difficult to be a long-term, buy-and-hold investor. Those were the Great Depression conditions that surrounded 1973 (no duh), and the 1973 bear market in which the earnings of even great non-cyclical companies (think Pfizer, Johnson & Johnson, PepsiCo, Coca-Cola, Procter & Gamble) collapsed. 2008 is an honorable mention if you were an investor in the financial sector.
With many blue-chip stocks cutting their dividends in half and earnings falling by at least that much (and prices falling by 75% or more), you couldn’t assess the present fundamentals of the company and see that the prices were irrationally cheap—maybe in the narrow sense that you saw that the prices plummeted more than profits, but it had to be weird to see AT&T go from earning $4.88 per share to $2.8 per share, Procter & Gamble go from earning $3.25 per share to $1.79 per share, and so on. If that’s what the dominant firms were doing, you can guess how the rest of a stock portfolio might have fared.