There's two answers.
1. In a vacuum it's a bad value trade. Assuming Jimmy becomes a true franchise or god forbid an elite QB, we were never going to replace him and a second will seem egregiously low.
2. Outside of the vacuum, the true context is Jimmy or Brady was leaving and Brady was playing his best stretch of football and amidst an MVP season so you were either going to trade Brady and resign Jimmy in the middle of that (or right after an MVP and Super Bowl appearance season) or let Jimmy walk. Or cripple the team for years to come by signing both.