If Cena went heel I don't think merchandising would be impacted much, if at all. The kids would simply gravitate to the next biggest babyface and buy all of that person's merch. Which, in this case, would be Daniel Bryan.
God, the whole "Kids wont by merch anymore" myth regarding Heel Cena bugs me almost as much as the myth that Cena is a merchandise seller and not just a bi-product of the WWE merchandise machine.
I really wonder how true it is, enough to offset the box office draw and PPV rates that say, heel Cena vs Bryan or especially Punk would make?
I've just not witnessed any organization just outright ignore how much their average fans hate this guy, especially when they pride themselves on being so dialed in on SCSA and Mankind back in the day. Hogan in the early and mid 90s was just tired, and it didn't help that he's feuding with Kevin Sullivan's gang of rejects. Cena is reviled. You don't get that many boos from just a bunch of smarks at shows.