Its so weird listening to people talk about Cena who've been fans of the product for years. I came in at like late 2013 and while I can see what people are talking about as far as him being squeaky clean and lacking any sort of Adam Copeland, I can't say he's been at fault for the product going downhill in my time at all.
He fought Brock Lesnar for the title in late 2014 and got destroyed. Twice. Then went for the US Title out of nowhere and his Open Challenge thing was legitimately the best thing RAW was doing at the time alongside the New Day, with the product being otherwise complete crap outside of his (and their) work. Since then he's just gone on to have a bunch of bad-ass matches with Seth Rollins and AJ Styles, and more than a few times has lost matches, and definitely lost his feud to AJ overall.
Plus, his acting career just started and now the guy's usually nowhere to be found. Can't hate a guy who's barely there.