Vince is senile and in his own world, he keeps Cena on top because he's lost the plot and thinks Cena is the answer to all his problems. Cena is terribly booked and overpushed, puts out terrible promos, and has instant win endings to all his matches. He absolutely sucks.
On people not knowing who Cena or Punk is, that's kind of their problem, not WWE's, Punk and Cena have been in the WWE longer than Rock was, or even Austin for that matter.
Like I said, Austin alone would bring people back, you said yourself you came back just for Rock. Do you want to see Cena acting like a dope while Austin internally sighs, or do you want to see Punk taking this seriously? You might not know who Punk is, but it's worth finding out.
The point of these matches is not to pop a big buyrate for WM and that's it. If you saw WM 30 with Rock/Austin IV, and whoever won, would you be back the next night for RAW? I doubt it, given neither man will be there beyond a week or two after the PPV. On the other hand, for example anyway, you see WM 30 with Austin/Punk, and find yourself being quite entertained by Punk, and so you might find yourself tuning into RAW the next night to see more of him, and before you know it you're getting to know others on the roster and liking them, then you're a fan again. Rock/Cena didn't do much of that, because Cena is an unentertaining goon, but do you know how many former fans returned after CM Punk's "pipebomb" promo in 2011? It was a lot.
I'm saying a lot on this subject but it seems important to me to not rely on the past so much.