Cena IS a draw but it's been proven in the past the show can also draw without him. When Edge won the WWE Title in 2006, I remember reading that ratings went up a certain percentage and then went down pretty much the same percentage when he lost the title right back. Edge also constantly drew the biggest rated segments on the show, I believe. When Cena was injured for a few months in 2008, the ratings remained pretty consistent. When Punk was a heel champion last year and the show wasn't just revolved around Cena, the show still drew. I believe the total averages in terms of viewership from about July to November/December listed Punk at #2 behind Vince, followed by Big Show at #3 and Cena at #4. Hell In A Cell, which had Punk and newcomer Ryback but not Cena, did good numbers, and TLC 2011, which had Punk but not Cena, did better than TLC 2012, which had Cena but not Punk. (Some might make the argument that Cena was up against an unproven draw like Ziggler, but then the same can be said about Punk the previous year being up against Miz and Del Rio. Don't forget WWE even blamed Miz the month before for Survivor Series 2011 doing not-as-high-as-hoped numbers.)
Obviously, if you took Cena off the show completely, it would probably show up in the long run, certainly in terms of merchandise sells. But I guess my point is that it's enough reason for WWE not to have strong fears with shifting gear with another top face for awhile. Not likely to happen since Cena just came off a big win with The Rock but still, WWE needs some fresh blood in the main event that to hopefully help make viewers tune in more. CM Punk was (and still is) kind of a person to fill that void and at the moment, Daniel Bryan is the best guy to put in that position as well (it becomes all null and void if they turn Bryan heel this Sunday, which would just be crazy.)
Also, PG isn't the issue, the writing is. Going back to TV-14 wouldn't automatically solve the problem, just like going from TV-14 to PG didn't.