Ppl, the man has earned north of $300M+ dollars. That's an extraordinary career payoff for a professional athlete. And unlike a lot of other players, he seemed to have a bunch of no trade clauses, which not everyone is privy to.
If he'd come back to the C's for Vet's minimum, and helped the new squad as a senior advisor, perhaps eighth man off the bench, then yes, he's a Celtic for life.
I mean the other Celtic greats, Russell, Heinsohn, Cousy, JoJo, Hondo, etc, orbit the Garden in some way, shape, or form. The big exception here would be Bird but that's because the el cheapo owner ditched him for Pitino. And others, like McHale, went back to their home state, which is understandable because there's no place like home. You could say the same for Bird but for him, that was his 2nd choice for an NBA front office career.
KG, however, is just following the money.