Dominant? Have you watched the decline the last 2-3 years? Those days are over.
Was there a decline aside from this season? Just a year ago the guy threw for 5300 yards and 43TDs winning 13 games. The year prior he won the super bowl. This year the numbers were down considerably, but he was on a significantly less talented offense. Oline was beat up all year, Godwin doesn't look the same post injury, no Gronk, Julio Jones is good in short stints not a full time player anymore, and they had the worst run game in football. Even with that mixed in between some absolute clunkers were some excellent performances as well. As these guys age it's not like they just lose it overnight, I think the bigger problem is consistency and can they do it over a full season week after week as the bumps, bruises, and general soreness don't clear up as quick.
If he came in mid-way through the season and signed with let's say a team like San Francisco I absolutely believe it would be impactful for that playoff race. It's always tough for a new QB to come in and learn the deeper pieces to how an offense works, gel with the receivers, get comfortable with the oline, etc but Brady is probably the most equipped to do so. You let him come in with a fresh arm and body around Thanksgiving, give him a couple of weeks to get right and yeah I can see him going on a mini run.