Actually, the whole 2007 season was a success for Thomas, and he followed that up with a great first half (9 games) in 2008 where he was headed for double digit sack totals until he broke his arm and went on IR. Prior to the unfortunate injury, he was doing very well in NE.
When he came back in 2009, he scoffed at the idea of changing his responsibilities which I believe included the idea of playing a lot more strong side LB, grumbling and making noise about it in the media. His attitude soured quickly that year, and I believe that he was part of the “late to practice/sent home early” group when it snowed in December.
Since they were now 3 full years into the deal and could see a substantial cap savings, Belichick decided that it would be best to part ways. Ultimately, his time in NE should be seen as a failure, but I think some get a bit carried away at times. The only year I blame him for is 2009 due to his poor attitude, but the LB group as a whole wasn’t exactly stellar that season.