While it is definitely something which supports your claim, when taken in context it isn't a huge deal.
For the following reasons:
- The total difference in sales for the XB 360 version from the 2013 -> 2014 was 466,193(2014) - 334,446 (2013) = 131,727 and PS3 version 340,240 (2014) -303,766(2013) = 36,474. Which is a total net gain of 168,201 copies or 4% of total sales.
- The price for a new copy at this point is $20, not $50 - meaning these sold copies are less profitable.
- This is one isolated case. There are many games that haven't greatly benefited from sales because of PewDiePew's gameplays. Notice that this video had 12 million hits (at the time of the article) and most of his videos only get 3-5 million. Which games this benefit is very unpredictable and almost like winning the lottery.
- PewDiePie is the most popular let's player, youtuber, and has celebrity level appeal. Certainly he can affect sales sporadically like any other celebrity. I wouldn't call most let's players celebrities though.
- Celebrities can play games on non Youtube platforms too and affect sales. Ellen and Oprah sold plenty of Wii's, for example. However, there are many instances where a celebrity's endorsement doesn't affect sales too much. This is true for both these mega-celebrities and gaming centric ones (PewDiePie.)
So considering how the potential gain in sales and profit is miniscule, and the risks can be high (Intellectual Property damage) it can possibly not be in the interest of a profit-seeking firm to have let's plays (look at the demo argument, often demos hurt sales and damage IP's, so many companies don't take the risk.) The overall opportunity cost could be less than the real-world costs, meaning profit was not maximized.