Stafford said he would have played for the Lions if they declined to trade him. They were a 5 win team with Stafford. They probably have two more 5-6 win seasons and he walks as a free agent. I fail to see how that helps the rebuild.
It also worked out well in that the Lions were able to trade up from 32 to 12 without having to give up much. A good draft for the Rams to win the Super Bowl.