I originally thought that Rock will help Reigns beat Brock and it will cement a heel turn for the Rock, then he will Rock Bottom Reigns so Rollins can cash in.
Now with him resigning, I can see two things happening. 1. He loses by some kind of cheating from his opponent or 2. He wins and says screw it on Raw because he is bored, and a little tournament happens, someone wins, Rollins cashes in afterwards.
The fact is Brock without the belt is more valuable to the WWE than anyone else holding or feuding for the belt. Brock does love money but I don't think he would re-sign if he knows that he will be just a midcarder once he drops the belt.