If your addressing CM Punk's turn as a ratings killer, you can also blame the other top guys constant face value as a reason as well.
Do you guys know why ratings were so high when McMahon was a constant character back in the late 90's? Edgy T.V. He had a wide array of enemies and allies, it was never done before, his rivalries and alliances were so top notch, so interesting and unprecedented that no one knew what to expect. Not to mention the people he was working with were, without a doubt, the hottest things in wrestling, period. Now when you tune in and see the same guy come out every Monday night doing the same thing for about eight years, consistently doing the same thing while the product continually declined into a state of mundane predictability, and other guys randomly being pushed and then the ball being dropped, or guys turning heel when they don't need to or guys turning face when they don't need to, its no wonder the ratings slip.
Vince will bring a spike of interest just because he is Vince McMahon, if he wants the ratings to be up above 3.1 or whatever it is RAW gets a maximum of these days, he needs to take a step back and evaluate the direction of his company, the position of his superstars.
I don't know if this is true, but I think if Cena were to have turned heel over Punk in a fashion that could have been booked as Cena "losing" to Punk and becoming fed up that he isnt the best anymore, I think its plausible to say interest and ratings could go up. I would be on board to tune in and see what a heel Cena can do, but instead they force a Punk movement and make him heel while Cena still plays the same old.
Maybe Vince figures that Austin's heel turn was more detrimental in the long run then it was for the short term, that it would happen to Cena. Who knows, but ratings slip because the product has slipped, no other excuses thats the truth right there. Nothing can be done unless they really want it to be done, and judging by WWE's outside interests, I.E Linda, Anti-bullying, complete contradiction in execution. Not blaming the WWE PG or allegiances and interests it has outside, but it has its hand in it.
They need to step the game up if they want ratings to go up. I'd take a half hour show of RAW if it was interesting and captivating over a 3 hour show thats ridiculous and predictable week in, week out