I think it's a combination of loyalty to the franchise + a ton of money. I don't think there were any other guarantees made.
From the Celtics perspective, I think Danny likes Doc both in a contending role and as the coach of a rebuilding team.
i remember in an interview, Danny said he liked doc because of his leadership as a coach and his ability to motivate his players. He also said that players LOVE playing for Doc
Yeah, other than some initial frustration by Pierce, I can't remember a single player ever griping about Doc Rivers, or doing anything other than singing his praises. IIRC, Ray and Pierce both made mention of Doc's return as being key to their re-signing last offseason.
If Doc walked, he would have had his pick of countless head coaching gigs. But, it seems he recognized how good he has it here, even if and when the team does go through a rebuilding process. He and Danny seem to work very well together; sure seems like there's a ton of mutual respect between them. He knows that ownership will spend money when it's called for.
Doc also seems to be very much a guy that *gets* and respects that tradition, and who *gets* how important the C's are to Boston.
So, he resigned to work for a great franchise, to work with players he loves, to work with players who love him, to work with a GM that he respects and who respects him, and to work with great owners.
Oh, and for a metric butt-ton of cash.
Sounds like a good deal to me!