i personally wouldnt miss him, i cannot stand him, as soon as he comes onto RAW week in week out and cuts his promo i want to kick him in the balls to shut him up, he talks about being 'against the odds' so much that phil collins should be singing his f'n theme tune
the only thing he is to WWE is merchandising, the only reason he was champion for so long was because he could shift the most t-shirts a month, but like its already been said, he didnt need to be champion to do that, look at CM punk, he wasnt ECW champion for ages, yet i bet he made the WWE a damn good amount of money just because the fans react to him well, if they put cena on the bottom of the card, and gave him two minutes to speak a night, the marks would be happy, we wouldnt all have to complain as much, and wwe would still sell out on all their 'chain gang' shirts.
i dont think hes a huge enough star though to be missed severely for years and years to come, what the wwe have doen with cena is what they couldve done with any wrestler on the roster, take jeff hardy, sure alot of people hate the guy, but the little kids go nuts for him week in week out, market him like they have done for cena and theyve got a replacement already, cena is a manufactured drone that wwe will milk until they drive him into the ground.
if he never came back though, he wouldnt be missed, he really wouldnt, sure his fanbase would be disappointed he wouldnt be back cutting another repetitive promo or having another stale match, but he wouldnt be missed. For example, Shawn Micheals is on a huge roll now, the fans are really getting behind him, and he must be getting the loudest pop of the night (or possibly behind jericho), if he came out tonight and said 'im retiring for good' he would be missed, fans would have signs out for weeks on end asking him to come back, so far i havent seen any signs to bring back cena, it is like alot of the crowd have already forgotten who he is.
which proves the point completely, so far i dont think hes been missed