I think the trade makes sense for both teams. Mack rounds out Chicago's defense that already has a large handful of young players (Howard, White, Trubisky, Roquan, etc.) and assuming Trubisky develops well, they won't have to add many pieces via the draft.
On the other side of the trade, I believe that Jon Gruden believes his team isn't built to win this year. He has a long contract so he has some time to shape the team how he sees fit. Mack would have been a great piece, but you would probably pay a premium for a player for 2-3 years when you aren't really expecting to compete for a Super Bowl. It makes sense to flip him at his peak to more adequately satisify future goals and expectations.
Both sides win. This is also a great side effect of giving a coach a long contract. They don't feel like they have to cut corners to compete now. They can be smart and ensure they are successful for a long time.