The term "holding people down" is something that I've never really understood. While I'm sure that Cena does have influence with the WWE brass, it's not ultimately up to Cena whatsoever who gets pushed and who doesn't.
In every wrestling company, you're going to have guys that are pushed and some that aren't. You're going to get some wrestlers that ultimately might not be as good over some guys that are better. You're going to get guys that think that they're better than what they actually are & complain that they're not being "used right". These aren't uncommon happenings in pro wrestling. Ultimately, pushing a guy to be the next big star isn't what you can call an exact science. The powers that be have to look at guys, evaluate the qualities they've got, their strengths, their weaknesses and ultimately decide if they're going elevate him rather than Wrestler B. Sometimes they make the right call, sometimes they don't.
John Cena is where he is because he's ultimately shown that he deserves to be there. He works his ass off for WWE, he genuinely loves working for WWE, he is actually a pretty decent in-ring competitor in spite of the often unreasonable amount of hate he gets from the IWC, he can cut great promos, he's loaded with charisma and, most importantly, he's able to get fans interested and invested in whatever program he happens to be working. I'm not even a huge John Cena fan, I like the guy pretty much and all, but nobody can realistically look at the facts and say that John Cena is a bad pro wrestler.
But as to holding people down, not really sure I buy into that because just look at the people who have benefitted from working programs or matches with John Cena. Randy Orton and Edge both made it to the main event level on a long term basis by feuding with John Cena. Dolph Ziggler had a few fantastic matches late last year against Cena that showed that Ziggler has potential to be a main event level star. Sheamus comes out of nowhere to feud with Cena. His career goes from curtain jerking to the main event almost overnight. The Miz's place as a main eventer is cemented during his feud with Cena. R-Truth has actually become a relevant force in WWE while feuding with John Cena whereas 6 months ago, most of us were hoping that Truth would be future endeavoured. And now, CM Punk is the most talked about wrestler in the world. A lot of it does have to do with the fact he's a great talent yes, but he also found himself in just the right program with just the right opponent and the right storyline. Come what may, I don't see how anyone can't see CM Punk as a main event level talent when his feud with John Cena is all said and done.