I don't think it's lack of faith in his own health as much as it is principal...
Jerry had an agreement with him, and now after giving his all to us (and getting hurt as a result of his commitment to excellence, in his own opinion) Jerry wants to change the terms of the original agreement.
Yes, I know, it happens everyday in the NFL... but I have always been a little mixed feeling-wise on whether or not I like the current system.
I like it because some hot shot can't have a killer season and then say "screw it" and stop playing hard because he got paid (i.e., Eric Dampier of the Mavs)...
But at the same time, as in Glenn's case, Jerry knew before he signed him originally that Glenn's knees have always been questionable, and certainly didn't get any better in all these years playing for us.
Anyone who's ever had knee problems (such as myself) knows that once you screw your knee up once, it's never the same after you "fix" them (through surgery or rehab or both).
Therefore, Glenn, in the spirit of principal, is sticking to his guns because he believes that his knees gradual wearing were in part due to his on field commitment to the Cowboys... and he expects the Cowboys to maintain the same commitment to him.
Sure, maybe another team would pay him with the same contractual clauses established... but he would be okay with it because that's the agreement HE is making with them.
In the case with Jerry, he is being told, "I'm tearing up our old agreement, and giving you another one.. .take it or leave it." And Glenn finds this unacceptable.
Although I selfishly want him to play for us, I can't be mad at him for sticking to his guns.
I guess this just kinda opens up a new thread at some point on our opinions of a new CBA.