Cena gets the bulk of the air time = Give time to the champ!
Punk gets the bulk of the air time = Give the time to someone else!
I agree they overexpose their top stars which is why they have zero high end PPV matches. Can you name one PPV match that could draw money we haven't seen on free TV yet? The booking is so incredibly top heavy that the main eventers get all the air/mic time, which consequently diminished the chances of midcarders getting over.. which continues the show to be top heavy. WWE is so afraid to eat a few weeks of potential bad ratings to plan for the future. Back in the Attitude Era they used to give so many midcard matches quick backstage promos that it helped build something other than in-ring images of the wrestlers, they seemed like actual characters. If you can build them in that manner you aren't stuck with "will this gimmick get over", rather you have "will this wrestler get over". One is much easier to accomplish than the other.