Let's put this to the test shall we? I have a graph here.
John Cena has been on Raw since 2005. Between '05 and the first half of '07 ratings remained relatively similar, if anything they increased. In the second half they dropped significantly. In 2008 the ratings bottomed out, and in 2009 increased significantly, before returning to late 2007 levels in 2010. In 2011 ratings dropped by less than 0.1, which I don't consider especially noteworthy. I don't have figures for 2012 and I cannot be bothered to find them.
In Cena's time on Raw there have been noticable drops on two occasions. One in 2007, fitting in exactly with the Benoit double murder suicide. The other in 2010, which followed an oddly high year, considering the ratings of the years immediately before and after it. Ratings dropping with John Cena is not a trend that can be determined looking at the numbers. They remain relatively similar or drop in isolated incidents.
Average ratings in 2012 were 3.0, I already posted it.
Here, watch this:
Ratings since John Cena first appeared have decreased - Fact.
Ratings since John Cena first appeared have increased - Bullshit.
You can come up with all the excuses in the world, you'll never change either of those statements.