I think both of you place entirely too much importance on wins and losses. This is professional wrestling, ok? The wins only matter in regards to who gets the rub. And as much crap as I give Punk for not having ability to put younger stars over, he did a fantastic job making Rey Mysterio seem relevant again. Hell, he made Festus and the bald chick seem relevant (even though all 3 went back to being absolutely nothing after he was through working with them, so he really didn't put any of them over). He was a serious work horse on Smackdown, and I as recall, most people were only watching Smackdown during that time for Punk.
His SES run was unquestionably the most popular angle in wrestling when it was going. Everyone on every forum had their luchador Punk avatars, it was booked very well. It's not like he was going to wear that ridiculous thing for the rest of his career either, so I'm not sure why you're acting surprised he lost it. Just because he didn't carry the World Heavyweight Championship for a lot of that time and was regulated to the mid-card scene, he was far from mid-card entertainment.
But yes, the SES stuff was getting old and it was time for a change. The Nexus angle wasn't very great, but Nexus was already getting old before he took over as their leader. He still maintained a solid heel status the time he was in it. It never once kept him from having a solid Wrestlemania showing either. Even Orton has been getting snubbed at the event in recent years. It's clear WWE didn't want to limit him to a career in stables, which is why he took off so suddenly after Nexus disbanded.
I just don't see how you can have so much faith in a guy that's difficult to build things around. He's proven he's not a great cornerstone various times. And don't blame Smackdown for not giving him a chance either because that's just lol. He took up just as much time on that show as he does on Raw. The only difference is he has a belt on Raw. And I guarantee you when he loses that belt, he'll go right back to the mid-card scene because people in the world title picture need to be able to put people over. That's the whole point of the title. He can do exactly what he does now with or without the title.