After booms come busts, and after busts come booms. Happens over and over.
Going back to 1820, stocks have never produced two consecutive decades of real losses. After a decade of losses (like we just experienced), the worst subsequent 10-year return we've seen is about 12% a year. That's a hefty return by any measure. [The chart doesn't bear that out. The 1880's lost -7.9% while the 1890's gained 13.4%. I assume that's total return, far from 12% a year. Remember that 2000 marked the peak of the bubble, so it's no surprise that the decade hence was negative. Didn't realize how negative. The Nasdaq I'm sure was quite a bit more negative. Remember it hit 5000 and now 12 years later it's still only 2700.]
Of course, history isn't guaranteed to repeat itself. And what drives stocks to a decade of low or high returns isn't the calendar; it's valuations. Stocks do well after they're cheap, and poorly after they're expensive. So the real question shouldn't be how long stocks have been stagnant, but whether they're cheap.
That's a matter of constant debate.