Well obviously nothing is guaranteed, but something like 90% of 20 year windows in the last 100 years would in fact result in positive real returns.

http://archive.nytimes.com/www.nytimes.com/interactive/2011/...

Note that the light red color is actually still indicating a positive real return, this is particularly relevant in the 70s and early 80s when a 2% real return would be a much higher nominal return.

Agreed that some people expect returns for time horizons as short as 10 years though, which is clearly a mistake.