it's not because of his play this past season, which was solid, not " great ". It's because of his contract.
Look at his salary and compare it to his play. You'll see he's making way too much for his contributions. If you don't believe it, then take off the T-new rose color glasses and ask yourself this question:
If the Cowboys release him today, how many teams would line up to sign him for the same contract ? If you know the answer to that, go ahead and name them.
Now, forget about next season. The Cowboys, because there's no cap, can easily carry anybody ( making anything ) they want. The question is what happens in 2011 ? or 2012 ? or even 2013 ( if by a miracle he lasts that long ), when T-New's contract stays pretty much the same, only his skills have diminished even more ?
Answer: the Cowboys will cut him and that's fine.
Only when they do, there will most probably be a salary cap in place, perhaps a much harder cap, and the Cowboys will end up regretting not ending his contract while it didn't affect it. I guarantee you that at least the fans in here would be screaming their hearts out asking why wasn't it done when the Cowboys had the chance.