I thought it would be interesting to break down the different sections of John Cena's fan base and debate if a heel turn would turn that section against Cena.
3 The blacks. They cheer Cena because they relate better with Cena's hip hop background. Most blacks don't know that pro wrestling is scripted so a heel turn would most likely cause this section of Cena's fan base to turn against him.

The most utter disrespectful, bland, incoherent and racist thread made ever just because of one word,"B*ack".
You had me till the second point but after you included no.3 , you lost all the respect for a dignified human from me. Do you think they are not literate enough to find out what is right or wrong? Are you stupid? You are an illiterate man! Everyone is equal and I think black men are more literate than the whites because they don't make racist threads like these. (I am white FYI).
Before this thread gets the racist of the millennium award, I'll answer your questions.
Kids- The majority of Cena's fans are kids. I see almost every kid on the shows or outside wearing CeNation's shirt and bands. It shows how the kids like and idolize him. I also see some of the mid teens doing that but they think they are smart by hating on Cena. Children may turn on Cena if Cena attacks other babyfaces like Punk and Orton but that's highly unlikely because Cena is their superhero.
Women- Women always like nice and handsome men. Cena is both of them and that's why he's popular among them. Most of the women are not favorable about the kayfabe terms so they may not like him if he becomes bad but they'll still do because he is a man! (Pun Intended)
I won't answer the third because if I do, I'd be a racist like you.
I'd like to add more on this. I also see many adults cheering for him. Especially those from 30-40 range. I know they do it for their kids but yet they wear Cena's shirts and come to the shows. Anti Cena fans are 17-26.
Just my 2 cents.