Markets have been slowly/steadily falling for some time. We ride that as that's the name of the game. I think I'm understandably concerned about another 2008.
I guess the eternal question, that is a matter of opinion as much as fact, is how you feel about riding a wave that might turn nasty.
If you're in your 20s then a 1929 style collapse is a buying opportunity as you'll hope to live long enough to ride the wave back to profitability even if it does take multiple decades. If you're in your 60s and the market tanks in a big way then it's far less likely you'll see a recovery.
If you depend on stocks for income then a collapse doesn't bode well - although your income would come from dividends rather than the share price directly, if the price collapses the chances are enough people in the market think the company is overvalued, and if it was expected to pay a good dividend the price would at least have some support (just to complicate things there are trading strategies like short selling, bear raids, bear squeezes etc, and sometimes markets move wildly around the time futures and options expire).
As a general rule the time to sell is before a major slump (for fairly obvious reasons), so the question then becomes one of whether you expect a further slump or a recovery. From what I recall (it's been a while since I read about it, so my memory is a little rusty) it was the second stage of the slump in 1929-30 that wiped out more people than the first stage. It seems reasonable to conclude that a lot of people figured the bottom was in and went in hard with their money expecting an easy return, only to find it nosedived again.
Personally I think we'd be better off if people didn't invest in things they didn't understand at all. Conventional wisdom is to put money into the stock market but so many people end up doing little more than handing it over to a relative stranger, trusting a few bits of paper that show they meet some government guideline, and then hoping for the best. I'm not entirely convinced that's a good strategy when it comes to something like retirement.