Awhile back, a video surfaced online of John Cena working out with trainer Mark Bell at Supertraining Gym in Sacramento. During the video, Cena proclaimed he will never turn heel when he said the following:

“[A heel turn] will never happen. Oh I’ve got it in me for sure…I do what they tell me, boss.”

According to The Wrestling Observer, there are many reasons why a John Cena heel turn is not “best for business,” and all of them are financial.

With Daniel Bryan injured and CM Punk gone from the company, John Cena’s merchandise is said to be outselling that of the rest of the WWE roster combined.

Additionally, WWE live events which feature John Cena average about $60,000 more than live events which do not feature Cena. Furthermore, in cities WWE has run several times in the past, shows in which Cena appears do much better than non-Cena events.

- It would come as a surprise to likely no one that WWE is resistant toward turning John Cena heel. Cena has been playing the good guy for well over 10 years. His character was later modified to drop the rapper gimmick as WWE moved into its more kid-friendly TV-PG era. 

According to a report from The Wrestling Observer, WWE is resistant to the idea of Cena turning heel because of the belief that it is not financially viable. As of now, WWE live events featuring Cena average about $60,000 more than live events without Cena. In cities WWE has done before, house shows that feature Cena always do better business than events where he does not appear.

In addition, John Cena's merchandise is reportedly outselling the entire WWE roster combined after CM Punk left the company and Daniel Bryan being currently out of action. On the flip side, one could probably argue that Cena turning heel and still being a fixture on the shows would not necessarily cause merchandise or house show ticket sales to plummet. When Hulk Hogan turned heel in 1996 it helped revitalize his career and the WCW product, since it became a big turning point for the NWO storyline. 

The other problem with the over-reliance on Cena and the lack of building up new main eventers is that it presents a huge problem when Cena is out of action or eventually retires. Cena is 37 and has already dealt with many serious injuries. How many more "miraculous" recoveries can his body take? When Cena is off TV, WWE rarely treats anyone with similar significance. The only reason Daniel Bryan received his push is because the WWE fanbase forced it to happen and rejected Batista's return. Who is to really say a Cena heel turn would be a financial flop unless you try it? It could allow another budding face to rise to the occasion as an equally powerful star. 

Credit: The Wrestling Observer via Wrestlezone.com 
Read more at http://www.411mania.com/wrestling/news/335140/WWE-Resistant-to-Turning-John-Cena-Heel-for-Financial-Reasons.htm#muxwsc5E3BXbrSRM.99

I think it should happen right before he retires. It won't make sense now, but he should have one heel run before he retires.

I don't really care so much if he ever turns heel, I just want to see him take on some sort of gimmick change. He's been stale for so long...

It's not that it doesn't make sense...it's the fact WWE is scared to do it. But like Near said, I don't care if he is a heel or face just change his stale character. As good as a worker that Cena is, his character may go down as either the worst or the character that stayed past its relevancy.