I suspect these sideways markets don't factor in the dividends. The famous 1929 - 1954 apparently doesn't.The folks here that think they can predict the future crack me up.
There have been 3 times that the stock market has moved "sideways", (not gone up). Once for a period of 25 yrs and twice for a period of 10 yrs.
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You may think dividends aren't important (and dividends in the US have been very low in recent years). But dividends have been much higher in the past.
Many of my (Canadian) investments are paying dividends in the 4 - 5% range. And that's enough to count.