Brock left because he was too weak to keep up with the schedule.
Cena had a good rookie year too. Debuted against Kurt Angle, PPV debut defeated Jericho who had just lost the Undisputed Title, challenged for the WWE Title on PPV.
The Shield have had an awesome rookie year last year. Wyatt's are having a good one too.
ADR had a good one: won the Rumble, MITB, the WWE Title.
Problem with those sort of Brock pushes is that the guy needs to be really ready for it all, and Brock wasn't, and neither was Sheamus thus why he failed in 2010 and had to be rebuilt in 2011 so he could re-enter the main-event in 2012 but that kinda failed too because WWE treated the World Title as second rate.