For months on end, Ryback has been dominate. He has defeated two superstars at a time and The Miz … that's pretty much it. A short feud with Swagger, two matches with the Miz, and a bunch of jobbers is on Ryback's resume. He's won no hellacious ladder match, contended in no over the rope match, and joined no tournament. He's just been simply dominant to the lowest of the low and a star or two. He's one dimensional, he has a catchphrase, and he marches like a robot toy. There's nothing relatable about him and there's nothing larger than life about him. Simply put: he's a glorified Brodus Clay or Lord Tensai.
CM Punk has been champion for over 300 days. He's fought Jericho for bragging rights of being "best in the world", he's battled all the ones to claim to being better than him, and he's faced against his current arch nemesis, John Cena. He's been a tweener, a face, and a heel in his run. His character has indeed run a course that would need a lose to justify another turn.
There is only one real way for Ryback to lose, and that's by passing out. Sure, it's been rehashed, but it still doesn't make it any less impactful. It doesn't make Ryback the next Austin, but it does hep him win over the rest of the fans. After a (what we can only hope for be) brutal fight, Punk manages to make the win by choking Ryback out with the Anocanda Vice. Ryback is no longer the dominant force that people believed him to be, and although he is no longer threatening, he is more human. He has become relatable for the fans. To do everything right and to still come up short. To lose that match you have dreamed about since forever. And now the fans will want to see you come back from that defeat to rise to be even bigger than what the fans perceived you of being. He would no longer be robotic, but human. He could make a speech that would finally give a connection with the fans. And Punk's reign would get a big win, next to defeating Chris Jericho at Wrestlemania for bragging rights of "best in the world". He did what the fans perceived to be "impossible", and now has all the talk in the world to go into his match against The Rock at Royal Rumble.
Ryback winning for a few weeks does nothing. His short reign isn't worth any sentimental value, and there's nothing in it worth ratings wise. Even if Punk wins it back, or someone else does, it takes away the value. Just like Del Rio's short reign in '11 and when Cena and Orton kept trading the title back and forth in '09. Ryback's reign will be worse than Swagger's world title reign in '10, and Del Rio's WWE reign in '11. It'd end up useless and not even worth the label of "transitional champion".