I won't argue with you on some of the points, but once Punk realized he would never achieve his goal of being the top guy in WWE, it's not hard to see why he became disillusioned. Punk's career goal was to be the closing match of WrestleMania, and that again wasn't going to happen this year. There are definitely some week's Punk phoned it in, but I am not going to agree with you that he phoned it in every week since Summerslam.
95% of the WWE roster is scared to death of doing something wrong and losing their job. Whether it be because of money, lack of other job opportunities, or ego. Punk doesn't have to worry about any of these. He is extremely frugal and has tons of money in the bank. He has an ego but not the type that requires him being on tv each week to be praised by fans. And if he still wanted to wrestle, he could easily go to Japan.
Most times I would be negative about someone being a "crybaby" and walking out on their job, but I have a lot of respect for Punk because WWE treats their talent like crap and thinks they can get away with it.