Yeah, but the QB is 26 years old and the Bears would finally have that position solved. It will take a couple years to build around him.
At that point, they're moving on from the aging defense via cuts and re-investing those cap dollars into the offense. Start re-building the D by trying to find undervalued players on the FA market and other ways. Plus, if Mack and Quinn are healthy they theoretically should still have a hell of a pass rush, even if they're weaker in the secondary.
They still have some long-term pieces on the D even without Smith and Johnson in Jackson(if 2018 wasn't a fluke) Nichols and Goldman. Mack is still good so even though he's paid a lot and is older, he's going to be here for a while.
2021 is going to suck either way, so why not just fix the QB position, which has been the Bears bugaboo for 70 years, and deal with the rest later?