IMO we let Teddy walk because his game was hardly different than "old man" Brees' game...high percentage short throws 99.5% of the time with an intermediate ball sprinkled in for good measure. I think CSP wanted a different brand of football, that's why he let TB walk without putting up too much of a fight.
I think it was more a matter of timing. TB would have had to sit for at least another year at BU salary if he stayed. He took the bird in the hand and the starting job in Carolina rather than wait for Drew to retire.