I don't think it's even a remote possibility. Not even a shred of a possibility. There really is no incentive for Belichick to make that move.
First off, there is no reason to believe his odds of winning in the next few seasons aren't still better with the Patriots than the Cowboys, even with Brady tailing off somewhat. And even if Brady retired, Belichick has a strong record in the games Brady has not played, including going 11-5 in a season where Brady missed 15 games.
Then there is the fact that he is pushing 70 years old, has made his home in the Boston area for 19 years and in either NY or Boston for the bulk of the last 40 years, is already considered the best coach in history and has more SB wins than any coach ever, is beloved in Boston and all over New England, is a lock HOFer, has full control over the team he coaches in New England (which everyone, including him, knows he wouldn't have in Dallas) ….
What exactly would motivate him to make that change?
And, one more thing … do we even know if he will be free from his contract with New England after this season?