I may not be too educated on the topic of exactly HOW Cena got his huge push initially, and I am a much larger Rock fan than Cena, and am die hard for the Rock at WM28, but it seems to me that Cena had quite the many fans behind him. I remember when Cena was still young on Smackdown. Didn't like him much. Liked his fued with Kurt Angle, but that was mostly because Kurt Angle rapped and said he's "movin on up like the Jeffersons". But I forget which Wrestlemania it was, probably 19, that after the night was over, there was a poll to the fans which asked "which superstar got snubbed by not appearing on WM 19?" And an overwhelming portion voted for Cena. It may have been after WM 20. But either way, the fans love for Cena did not come from the fact that he told Vince he'd be happy to step all over other superstars to be the top guy. Wether he has or not is another issue. But it seems to me the fans latched onto him early and strong, and thats hard to do, and I'm sure Vince saw something in him that made him think "this is my guy". Although this was also around the time of Lesnar so who knows? But to the topic at hand.
I absolutely agree if Rock had stuck around, Cena would not have gotten as huge as he is... at least not as fast. He would have been big. He would have been Main Eventing. And eventually Rock staying around would have gotten old, and you can only be the top guy for so long, not 12 years (unless Cena stays on top for longer, but it already seems as if he is falling, not fast or far, but falling none the less). But look at HHH. He was never the "top" guy, but he was surely one of the top guys. And now he's mostly in the back, still comes out every now and then, will fight taker at mania and help push (or bury, depending on who you ask) Punk, but mostly he's just "around". And I feel if Rock either didn't make it in Hollywood, or didn't go, this would be his role if he was still around as well. Not necessarily the "corporate" guy, but just around, to help push people (Cena). Also Cena may not have gotten the "hustle loyalty respect" gimmick. If Rock had been around at the time there wouldn't have been a need for a Hulk Hogan babyface character. So its very hard to tell, but I do feel even with Rock, Cena would be big, just maybe not quite as big, quite as fast.