When it come to the indices on "what goes down doesn't always go back up", yes it does. That is inherent in the design, because of the world economy. For individual companies, no.
The nikkei 225 still hasn't regained its peak from 1989.
It also matters whether it's going to come back in your lifetime.
It took what 25 years to come back from 1929? And that's just to get even. I imagine most people would like some return for tying up money for 25 years.
