Let's be fair...all of Cena's movies are WWE Films and really have been flops (like someone else on here said). As far as his countless appearances on TV shows, there are 10 as a performer on a TV series or movie and around 30-40 as himself...
CM Punk? I doubt he has any...but I'm saying that of the two guys, CM Punk has that style that could potentially transcend media. Remember, Austin's "Austin 3:16" speech didn't immediately propel him to the main event and to superstardom...it took around 2 years for him to reach his peak. Cena has been on top for a while, but just doesn't have that same appeal as the megastars (Austin, Rock, Hogan) do. In fact, a lot of his appearances were like Hannah Montana and the Kids Choice Awards. It's very similar to 1996 when Shawn Michaels was "the guy" and appeared on things like the Jenny Jones show. It was nice and it was exposure, but it didn't make him a megastar, just a star.
(Note: I got all of the info from IMDB.com)
Cena's films have been flops,but all WWE-produced films have been flops.Actually,so many wrestlers have acted in massive flops,regardless of their popularity.Even a true megastar like Austin couldn't parlay his popularity into a successful film career.Hell,fuck Austin,even Hulk Hogan couldn't do it.He had a nice cameo in Rocky III and then...Suburban Commando and Santa With Muscles.The Rock's the only true ex-wrestler turn movie star,and he had to give up on his wrestling career,turn his back on it almost completely and go down the Vin Diesel route to achieve his success today.Also,the success of a film depends on many other factors other than who's starring in it.The director,the script,the actors of course,the marketing (which for John Cena's films were nearly non-existent considering the WWE can't provide a marketing platform compared to a movie studio like Universal),the pre-release buzz,the number of screens the movie's showing in,to name a few.It's like how some people will watch a film because of the director (Woody Allen,David Fincher,Martin Scorsese,etc.),or because they hear that the movie's an Oscar contender,or because Aaron Sorkin's written the script.Many factors come into play.Also,Cena's definitely not as popular as Brad Pitt or Leonardo DiCaprio,but neither is Stone Cold or Hulk Hogan.
Enough about his film career though,there are many factors pointing to the fact that Cena is as popular as the WWE says he is.His album went platinum,for instance.I think it says a lot about someone's popularity that they can come up with rather mediocre music and still go platinum.An album's more of a solo effort than a film.Put it this way,you buy a Bob Dylan record because you like Bob Dylan.You watch Transformers because of robots,explosions,CGI fight scenes and Megan Fox.Add that to his TV appearances and I really can't think of anything that suggests that he's not the number 1 guy in the WWE and is their biggest moneymaker.
Also,the WWE consistently draws in ratings that are around 3.0.Those are pretty good numbers for a cable television show.Even a 2.8 rating puts them in the top 10 for Cable TV shows.I find it hard to label the WWE as a "dying empire" when other shows aren't exactly blowing them out of the water.TV's just become less popular as the Internet becomes more popular.
As for the OP's claim that Cena doesn't sell as much merchandise as the WWE claims...I don't know what to say to that except "God damn,man,watch an episode of Raw or two." Just look at the crowd and count how many Cena shirts there are compared to,let's say,CM Punk shirts or The Miz shirts.Cena has more than his fair share of shirts,not only on children,but adults too.Sure,CM Punk's been selling more t-shirts in the last 2 months,but does anyone really want to start counting how many t-shirts Cena's sold overall compared to Punk?He's been selling shirts like hotcakes since 2004 with "Ruck Fules".Punk threw a pipe bomb that sent a shockwave rippling through the industry and they're promoting him accordingly.He's been in the main event ever since,but Vince would have to be a terrible businessman to toss his biggest moneymaker by the wayside to get behind a guy that's pretty much riding on a wave right now.The wave may be as tall as the mountain currently,but sooner or later it's going to crash.The WWE would be willing to get behind Punk if he stays on the same level for a year or two.Getting behind Punk because of 2 months of solid shirt selling would be like getting behind MC Hammer because of Can't Touch This.The WWE's struck gold and they're digging for more.That doesn't mean they have to close off the other,bigger gold mine.This kind of monogamist thinking is simply naive.Besides,the fact that Punk's supposedly the biggest thing in wrestling right now and ratings are still stagnant is a BIG sign that he's not as popular as people claim he is.Either that or about 2 million people were about to simultaneously switch off their TV sets and never watch Raw again before Punk came on and did his "shoot heard around the world" promo.
Now I can't argue that Cena's as popular as Stone Cold or Hulk Hogan in their heyday,because he's not,but to claim that Cena's not as popular as the WWE says he is is preposterous.Cena's the biggest thing that's happened to the WWE in the last 6 years.Nobody's made more money for them than he has.
Lastly,this has nothing to do with Cena's popularity,but stop using the "Cena is boring and we're bored of Cena" excuse.You know what's boring?Paper.You know what I don't talk about or think about or watch?Paper.I don't go on forums and talk about my hatred of paper to paper aficionados or fellow paper haters.The fact that you're all still talking about Cena means that you're far from bored of him.People who go "Cena's boring!" and yet talk about him incessantly are like closet homosexuals pretending to be homophobes.You're curious,you want to try it,but you're so afraid of society's perception of you that you hit out in the other direction to seem "normal".