I have to staunchly disagree
From about 08 - 11 yes the character was stale. But I can't remember anybody Vince hasn't left stale, because he's just not qualified to produce weekly television anymore. If you want to talk about the intangible "X factor", Cena has it. He's a clumsy wrestler who dresses like a 5 year old to the ring and has somehow made it work for 15 years, putting on more great matches than Stone Cold, Rock, and Hogan ever did. No doubt he was corny but he was an actor on a produced TV show, you can't only blame him for stupid shyt but then hold Vince accountable for all of the other creative shortcomings.
From 2012 after he was doing the best work of his career and turned into this mythical figure who defied conventional wisdom. Even from 05 - 07 he was earning stripes as a top guy and having great matches on the regular. Did he tune people out? Most casuals left in 02 and the internet beards who cared enough to pop blood vessels over Cena didn't actually go anywhere.
Cena made a career out of turning feces into gold. Stone Cold and Rock had success built in to their characters. Cena was a gimmicky rapper turned stupid Marine Superman who wrestles in jorts, and still carried the decaying product for 10 years.
Call the last transcendent star of the business a "C list talent". The ratings were declining for years before he became a top guy. There's also no logic in holding both the entire success and failings of the company accountable on one performer.