He's just been so overexposed it's impossible to have any emotion about him anymore.
Compare it to Hogan's WWF run. Hogan went 10 years (1983-1993) but that obviously is a different animal given the lack of TV, but regardless 10 years. Moreover, you can just about trace the point when he was fading as a top draw for them around late 1991, early 1992 so under 10 years, right around where we're at with Cena right now.
It's just too much. The best thing he ever did was go away for 5-6 months in 2005 (I believe). He returned early at the Rumble and the pop he got was huge because it was nice just to not see him for a little while. He'd benefit SO much from having 6-7 months off right now. Every movement of his, every word, every move, etc. we've seen it time and time and time and time and time again.