I presume we're talking this hypothetical scenario after the 2019 season. Belichick leaves and Josh McDaniels is promoted to HC. Subsequently they may or may not have hired an OC or just promoted someone internally but ultimately the offensive approach wouldn't have changed much. Regarding personnel I'll just assume we're swapping out Newton for Brady with everything else the same.
As mediocre as their passing offense seemed at times in '19 they still finished as the 8th ranked passing offense. In '20 with a relatively similar set of receivers their passing offense dropped all the way down to 30th with Newton. Their overall offensive rank was 27th.
In Brady's 18 seasons as the starter the offense finished below 20th only once (21st in 2002 when Brady also led the league in TD passes). 12 of 18 seasons they finished top-10 in total offense (Y/G). They did however finish top-10 in points per game in 17 of 18 seasons (the lowest mark being 12th in '03). In '19 they finished 27th in points per game. All this meaning Brady obviously makes a BIG difference.
Looking at weeks 4-8 of '20 they went 0-4 with opposing quarterbacks throwing only 2 TD's against 5 INT's. Conversely however their own quarterbacks (Newton, Hoyer and Stidham all seeing action) threw only 1 TD against 9 INT's. Brady throwing lefthanded wouldn't put up horrific numbers like that. He's more likely to flip those numbers entirely, so with the MUCH improved quarterback play of Brady against the same poor play of the opposing quarterbacks, the Patriots are winning at least 2 of those 4 losses but more likely 3.
Looking at weeks 14-16 of '20, all losses, here again the NE quarterbacks did absolutely nothing with 0 TD's and 1 INT. One the of the opponents in this 3-game losing streak was Buffalo so considering Brady's .917 winning percentage against them it's pretty safe to put that one in the win column. I also don't see Brady losing back-to-back games against Goff and Tua so I would flip one of those losses as well.
End result would be a 4-6 game swing in '20. Splitting that difference they would have finished 12-4, coupled with Buffalo's record consequently dropping by 1 or 2 games, NE would have won the division and been the 2 or 3 seed. Also under this scenario (of Brady staying in NE), Tampa Bay likely wouldn't have even made the playoffs in the NFC leaving Green Bay as the clear favorite to get to the SB. Who knows what would have happened in the AFC with NE in there but you probably would have had a NE vs KC rematch at Arrowhead Stadium. Brady having to beat Mahomes and Rodgers for a Super Bowl title... that sounds familiar.
As for this past season, swapping out Brady for Jones (who NE would not have been in the position to draft so he's out altogether in this hypothetical), there's no way the team's starting out 2-5 and finishing 1-4. Week 1 vs Miami is a win. Week 4 vs Tampa Bay with probably Jameis Winston is a win. OT loss to Dallas probably goes in the win column. Without looking at it too closely, the 1-3 regular season finish is probably at least one game better. Add it all up and they would have finished 14-3 atop the East and the 1 seed. Potential SB 53 rematch... coin flip. Rams are a better team than they were in '18.
Most people probably will argue that swapping out Belichick with McDaniels would result in 1 or 2 more losses per season. Even conceding that, McDaniels with Brady is finishing no worse than 10-6 in '20 and 12-5 in '21. If it played out that way then likely they would have missed the playoffs in '19 but still won the division in '21 with possibly retaining the 1 seed.
