He's 29 (30 next year), already made the Finals, and gonna keep contending for the rest of his prime in BOS, a big market, where he made his place and gonna get paid.
I don't see him coming back in a rebuilding, NBA bottom team who's gonna fight to even make the POs, then maybe start contending in 3-4 years at the very best, just for the sake of playing with Wemby. These guys have their career too and staying in BOS is way much better for his.