Cena wont turn heel until a Wrestlemania much like Austin, the difference being that Rock, HHH, Taker, Kane and Angle were all selling lots of merch etc. at the same time. Therefore WWE could afford to turn Austin.
Cena sells a shit ton of merch everynight and WWE know this, until 2 or 3 other guys get majorly over and start selling merch Cena will remain exactly where he is right now and has been for the past 5 years.