I remember Scott Pioli being quoted before that he felt it was as big of a challenge to maintain a Super Bowl contender over the years as it is to build one from scratch. Maybe more so.
My guess is that both he and Belichick will ride this puppy until the thing falls apart or they get tired of football and need a break (ala Bill Cowher). Belichick probably looks at Parcells and sees that he never actually duplicated the success he had with the Giants and wonders if Parcells didn't bolt from the Giants whether they could had gotten one or two more championships.
Also, he probably remembers his Cleveland days where the owner totally threw him under the bus and figures he has it good with Kraft.
I don't get why people think coaches like Belichick long for going to another and creating a championship. Some coaches may like that challenge, but Belichick might like the challenge of remaining on top more. You didn't see Bill Gates leave Microsoft and start up a new company when his software dominated the PC market. Some guys can just see challenges and accomplishments in building on your success than seeking new challenges.
Don't forget if he wins three weeks from now, he will in position to go after a new challenge - the most Super Bowl wins by a head coach. I think he will be tied with Noll, but he probably wants sole possession.