Hm I understand Grudens logic, he thinks this year he can't win much, so he can spend every penny on Khalil Mack or trade him and build a new better roster.
Gruden isn't a Carr fan from what I get, so after all, he will try to get his QB next year in the draft, get an actual O-line that keeps the QB up and a running game and a mediocre to good D.
He needs a DE/CB/S/LB/DT/OL/RB and QB .. now he needs another LB ... but with spending ~160 Millions he wouldn't be able to get anything done.
You don't win SB with 1 guy expect for a great QB maybe ... all it takes is a team, if the team is mediocre at best, you want get over the hump with 1 player.
So from my standpoint, I can understand the logic behind the move, will the common fan understand that? Maybe.
Was it the right move? Maybe, all depends on Grudens/McKenzies drafting in the next 3 years. If they hit 2-3 home runs, No one will remind the Khalil Mack trade as bad.
In the End the NFL has many stories that go both ways, the Lambs didn't draft that good with the RGIII trade but decent enough to be now contenders, but we will see how this story ends in 2-5 years, if the Bears don't go anywhere in the next 4 years, the winner is Gruden by default.