I don't think Isaiah is a max player in the open market, however, he probably does deserve to be paid as a max player in Boston. It is one of those things where he is more valuable to Boston than any other team. A good example of this would be Carmelo Anthony in New York. And just like Carmelo, once Boston gives Thomas a max deal, he most likely becomes untradeable.
Maybe I missed the memo, but other than Chris Paul, how many starting PG's actually play defense? Yes he's only 5'9", but I don't recall seeing too many times where someone tries to post him up. It is pretty much a scoring point guard league now, and more often than not, Thomas is on the positive side in the scoring department versus his opponent. On offense, he's constantly putting pressure on the defense by attacking the basket and putting the opposing team in the penalty. And he is also a [dang] good shooter from deep, mid range, pull ups, and the free throw line, which makes him an offensive nightmare.
Thomas has so much good will from his teammates and the fans, that it would be almost impossible to just let him walk. Some may disagree, but I believe leadership matters, and Thomas just oozes that quality, especially since it seems as if his teammates believe in him.