I don't think WWE ever recovered from the losses of wrestlers such as Austin, The Rock, Chris Jericho, Brock Lesnar, Kurt Angle and Eddie Guerrero (amongst others) - huge personalities and capable of evoking huge amounts of heat or sympathy with ease. Jericho, Angle and to a lesser extent Eddie were also very capable of putting-over new talent - how many new careers has Jericho helped start within the WWE?
I think this is the issue Cena has - he's got to the point where he cannot really lose any small matches without things like interference, whilst retaining a persona that is hugely annoying.
Dont forget Flair, Benoit and to an extent, Booker T
two of the most entertaining feuds for me was Benoit v Angle and Austin v Booker T. The fight in the supermarket was hilarious. "Price check for a jackass!"