Ah, finally an interesting question involving John Cena and his role in the WWE. Props to the OP.
I was actually toying with a similar idea the other day, what if Triple H was never around. The thing is, as any true blue nerd would tell you, if you change one thing in the space time continuum, everything could change. By taking Cena out of the picture, the WWE may have kept RVD or Booker T around longer. Maybe Bobby Lashley would have never left. Maybe they would have found some random guy who has the ability but is stuck hopping around in independents.
But for the sake of this discussion, let's just ignore that space time continuum bullshit and talk about what the WWE has now.
I see 3 wrestlers who could potentially be the face of the company.
Triple H- Arguably did carry the company in the mid 2000's, though not always as a face, the King of Kings is definitely one of the best that the WWE has today, if not of all time, and while I do enjoy John Cena, I find Trips even more entertaining. Triple H has the combination of mic skill, in-ring ability, and experience that would allow him to carry the company. While he's sort of old for the face of the company role, I think he'd still manage to do it.
Edge- One of the coolest guys to ever grace the WWE ring. While Edge's most memorable work comes from his time as a heel, he was undoubtably a very enjoyable face. He doesn't quite have that clean-cut face look that John Cena hasEdge is covered with tattoos and his "flowing blond locks" don't exactly scream face of the company in this day and agehe definitely could have been a major face had his career turned out differently.
Jericho- Similar to Edge, some of Jericho's best work comes from his heel work, but in my opinion, Jericho is at his very best when he delivers promos screaming at the top of his lungs so he can be heard over the thousands of shrieking fans. Unlike Triple H and Edge, Jericho's move set is flashy, which may put him at an advantage over the likes of Hogan, Austin, and John Cena, all of whom have fairly simple move sets. Furthermore, Jericho can bridge the gap from Attitude Era fans to PG Era fans. Attitude Era fans will remember Jericho for his hilarious and insulting promos, while PG fans know Jericho for being able to rile up a crowd and really working a match to it's fullest extent. Name the last bad feud Jericho was in. I can't think of one. All these reasons make Jericho one of the best in the business, and it also makes him a good candidate for face of the WWE.
Other candidates include...
Orton- I don't like Orton. He's as unimpressive in the ring as John Cena, but unlike Cena, who makes up for his lack of variety with intensity and brute strength, Orton seems more casual and is not nearly as strong as Cena. Furthermore, Hogan, Austin, Rock, and Cena are all known for giving passionate promos that get the fans out of their seats and cheering for more. Orton's promos can be good, but none of them are inspiring, rousing, or even that entertaining. At best, his promos are interesting. The only reason he is even being considered is because he is so ridiculously over right now.
Bobby Lashley- I don't know why he left the WWE. I'm sure a trip to his Wikipedia page would provide the answer, but I'm too into this post to interrupt it. If John Cena never came to the WWE, there would be a wide open spot for Mr. Lashley to fill. I don't think his rise to stardom would have been as impressive as Cena's (in all honesty, it's because he's black and we all know black superstars have had limited success in the WWE), but he has the build and the moves to make it to the top. If he could get some good mic skills, he would be able to do it. There are only two things holding him back. 1) He left the WWE. 2) He couldn't talk worth a damn.
To wrap this already terribly long post up, I don't think the WWE would be where it is today without Cena. I don't think any other superstar has the look or talking skills to attract public attention like Cena has been able to. WWE would have no real star that could suck people into the WWE, and they would suffer a great decline in viewership, thus further dollars from advertisers, and thus a general decline in product. Love him or hate him, every WWE fan should be thankful for Cena.