I'll start this post out by saying that there is a book predicting this price target. You can buy it here: Dow 36,000.
I have not read the book, but will at some point in the future. I want to mention that the markets would have to increase by around 157% from today's prices. Putting this into context during the 1980's and 1990's the markets trajectory was even larger.
The 1980's had a gain of 228%
The 1990's had a gain of 316%
The 2000's had a loss of 10%.
The 2010's ...........................
The 2020's ...........................
Where will we be in 10-20 years? I do not know, but I think we are on the verge of a massive global expansion. Does it start today? I'm not sure, but the use of the internet, mobile communication, and global commerce is nothing to sneeze at. We are currently trading at all-time highs and if the markets continue to slowly push higher, these questions are not of out the equation. If the past is any lesson, we'll probably have 10-20 years of prosperity, before another major market crash like 2008! Be safe!