Honestly I don't think Phil sold out. I actually think that prog wasn't exactly his thing all along but that pop music really what he liked to make. Judging from interviews I've read from the PG era, some of Phil's comments made it sound like he was just doing his job as the drummer (albeit a great job) and that the music they were making wasn't his cup of tea.
Mike and Tony are probably more of a sellout than Phil. "All I need is a miracle" is pop-perfect, and although I really haven't listened to Tony's solo stuff, his contributions to latter-era Genesis were very popish. The thing is, those two were the cornerstones (although held in place by Steve and Peter) of the prog sound of early Genesis. This is further evidenced by some snippets of the last album, CAS, when I think a few glimpses of prog can be found.
anyhow, again, my opinion is that Phil always was more pop than prog. After all, The Beatles was one of his biggest inlfluences, and they were pop way before they moved into any other type of genre. However, it is sad to see the quality of his work go down the tubes after his first 4 albums were so awesome.