Everyone has pointed out the issue with the monthly vs. yearly and the 20% calculation instead of 10%. But there is one more problem.
The stock market will most likely NOT grow at 11% per year in the next few decades. Lets look at your cherry picked numbers. I say cherry picked because you used a period that:
(1) Avoided most of the great depression fall, that is you didn't start in a region from 1928-1930 when the stock market was far above the 1926 levels.
(2) Avoided the stock market crash and further pause from 2000 onwards.
Given that, I will use your dates anyways. You didn't specify which part of the years 1926 and 1999, so I'll pick the beginning of both. The Dow Jones in Jan 1926 was about 155 and in Jan 1999 was about 9400. So lets do the math. The return just on the stocks was 5.79%. Why? Because 155*1.05785^73 = 9400. But stocks pay more than just the gain in the stock market price. They also pay dividends. Dividends WERE running about 5% on average. Thus, you GOT 5.79% + 5% ~= 11%.
However, things changed in the internet stock boom era. Dividends were ended or slashed. Now, they are closer to 2%-3%. Thus, a total return of 8% is what many experts would predict in the future (going off of the past 5.79% return on the stock price). The real picture may not be as rosy though. That 5.79% number includes years when baby boomers pretty much added money non-stop and never took any out. But, starting very soon, they will mostly all retire. A massive bunch of people will suddenly switch from adding non-stop to withdrawing non-stop. You'd have to be pretty naive to think that the 5.79% number will continue. Realistically, we may be getting lower than 11% and lower than 8% return for the next few decades (until the baby boomers die and stop selling their stocks).
Your real math should be more like this: assume 7% return yearly over 40 years. Thus, the $200 becomes $200*1.07^40 = $3000. Inflation adjusted, that is just $800 worth of goods or services in today's dollars. If you are lucky and do get 11% yearly returns (and assuming there aren't bad inflation years), you'll get $3600 in inflation adjusted money in 40 years. $3600 is a lot. But it is a far cry from the $200 billion that you claimed.