The thing about Belichick is he got fired for failing as GM, not head coach. He kept overruling his scouts and player personnel folks to take offensive scrubs that busted over and over, and by 2023 the Pats had arguably the least talented offense in football.
Worse, he could never admit he was wrong, even privately, so he stayed dictator over personnel right to the bitter end.
Belichick could come to Dallas and be a coaching upgrade, especially on defense. There's enough offensive talent already, so maybe that's OK in the short term, and he can get the maximum out of the defense. But either he gives up on being GM and you didn't fix the biggest problem the Cowboys have, or he takes over as GM, and you've traded one kind of personnel failure for another.