The fans that complain are long time fans who loved the attitude era and now that things are quieted down, they demand more exciting content. WWE is directed to families mainly and not to teenagers+ specifically like in attitude era and the long time fans don't like it. John Cena is just a scapegoat who plays as the squeaky clean baby face of the company but is most popular. Cena recieves alot of pushes from wwe because of his popularity among kids and thats why wwe made him champion alot of times to satisfy the kids.
Cena can be more entertaining for the long time stuck in attitude era fans if wwe had pg14 by swearing and sexual reference (like the rock) or w/e which they can relate to.
I think wwe needs another attitude era to wipe tna impact off competition before tna becomes an real threat like wcw did. Most long time fans think that as well.