In terms of bad things happening, that's fairly benign. It could be worse, and probably will be some time. I'm just not going to predict when.
During the intermediate years you would have spent a great many years looking at absolute carnage. Periodically getting trolled by bear market rallies.
"Barely kept up" is therefore very theoretical, because few people have the character to wait it out that long. Many who call themselves long term buy-and-hold, will quietly change their designation after 10 years of watching blood stains drying up.
That's exactly why, after such periods, markets are not just cheaply valued but almost literally "empty" and deserted.
Btw, we haven't had one of those since the 60s/70s.
Investing in the mid- to late 20s generally, a period of overvaluation, you would have done well initially. But for people who didn't have perfect timing to cash out (aka most shmuks like me), every buy-and-hold dollar invested in that timespan would remain deeply in the red for a very long period of time.
Due to the severity of the actual 1929 crash, a brief moment of semi-good investment returns (measured over multiple timespans from a few years to decades) occurred in 1933-1935.