I can not wrap my mind around the concept of "franchise player".
Having said that, loyalty is everything...if you trade a long time player, you are not really improving the team because you are damaging trust and chemistry of the other players.
I believe Kevin Garnett has two reasons to come back to the Celtics. 1. Win a championship. 2. Play with Pierce, Allen and even Rondo again to win it together.
Everyone knows what a loyal guy KG is and everyone knows he loves playing in Boston. However, I am afraid if you break up the core he will be more tempted to retire. At this point, it would undoubtedly be difficult to bring the the big three off the bench, but it has to be done, to prepare for the future while limiting their minutes...maybe changing up who starts from game to game depending on match-ups. Danny has already started collecting pieces for the "new" Celtics (Rondo, Bradley, Green) and if KG and RA will both come back at a discount, we will continue to be contenders while rebuilding.