So which is it? We need a great QB? Or it's a team sport and you don't need a great QB? If it's such a TEAM sport and you need a great defense then what is so bad about trading a player to reinforce the defense/team?
The same people are saying Brady has 4-5 years left and simultaneously the defense is years away and that it's a team game. Well then.
False dichotomy.
Football is the ultimate team sport where three separate units of 11 men to a side have to choreograph effectively to win the game. You need a good overall team to win the super bowl. Special teams, defense, and offense.
Having an MVP caliber QB is the biggest single piece to creating a overall winning team, beacuse an elite QB is the biggest piece to an offense. Thus one player can make the biggest possible contribution to 1/3 of the game. So trading such a QB is non-sensical if your goal is to create a winner in the short or medium term. Especially when your likely return is unknown draft picks.
You need a good team to win, but you also need some luck. You need to make the 50/50 plays, you need to be healthy, and you need to play well.