#1 There are more deserving Superstars
Finally, the biggest problem with CM Punk returning and winning the Royal Rumble is that it will take away another opportunity from younger and more deserving Superstars who have worked hard to establish themselves.
While CM Punk returning and winning the Royal Rumble would be a dream comeback for the Chicago chap, it would deny an opportunity to someone like Kevin Owens, who has been making his way back to the top of the mountain and would benefit hugely from a Royal Rumble victory. Even someone like Drew McIntyre deserves a major push for all the hard work that he has put since his return to the company. They need this win more than CM Punk does!
Ultimately, it comes to the point which many fans have raised heavily in the last few years - WWE should focus on the future and try to build more top Superstars who can carry the baton forward. NXT has been successfully doing that and in turn, is getting praise and admiration from fans. From what we know of Punk, he loves the business and wants to see it succeed and I'm sure he would agree with this point himself!