That's a different issue. I think what @lintiness is trying to say is that just because a market went up in the past does not mean it will in the future. Survivorship bias is absolutely huge here.
Most people forget this, but if you look at global markets since the early 1900's, there were several that went down and never came back up. Germany's markets failed on at least one occasion and never came back. A good paper that addresses part of this is here: http://www.researchgate.net/publication/4913017_Global_Stock...
Depression and war do many things that create discontinuities. Most people don't come close to thinking about those because they aren't knowledgeable enough to even understand how strong the survivorship bias in their current local market like the US.