Cena still isn't half as mainstream as Rock was in 2003, even before he became a full-fledged Hollywood actor.
I see Cena as being more on DX's level during the Attitude era. His fans like to do his hand gestures and repeat a few of his lines in public when opportunity rises, and he's many people's favorite wrestler. But he's never had the overboard raving cult following Rock has. Hardly anyone is cutting up and a having a good time the next day with their friends by just talking about Cena and not many people feel like they're "cool" to be a fan of his. Clearly some do, but again, it's not anywhere near as mainstream as Rock-worship used to be.
Some people will say "being cool has nothing to do with it", but it has everything to do with it. In 1999, if you had your own website on Angelfire or Tripod or Geocities, people thought you were a nerd. 5 years later, if you had the same thing on Facebook, you were "cool". Rock was "cool". So was Steve Austin, Hulk Hogan, and many other immortals. Cena isn't. Of course not all the blame goes on Cena for that, a lot of that has to do with the time and the current state of the wrestling industry, much like my comparison to social networking. Social Networking is "in" now, so now are a lot of sites like Tumblr, Twitter, Instagram, and hundreds of other sites that do basically the same thing Angelfire, Tripod, Xoom, and Geocities did in the 20th century... to give you a sense of individuality on the internet. Wrestling really isn't the fad it used to be so Cena could never match Rock's level of popularity. It won't happen ever. Cena will likely be long-retired before wrestling ever reaches another golden age where it's widely-accepted by the general public again, likely after under-going several metamorphoses of not just the wrestling product, but society in general. Maybe one day MMA will become frowned upon for whatever reason, maybe after the next GSP does something that makes Chris Benoit's murder/suicide look like jaywalking. Who knows. But right now the "cool" thing to watch for 1-on-1 gladiator-esque action is MMA, not wrestling.