The same people who think it's nonsense today probably would have said the same thing for the completely opposite reason after the Washington or New York game. (Justin's trade value is now perceived to be really high considering his ceiling, while earlier in the season it may have only been a mid round pick)
To actually answer your question though, while we have the cap space to pay Lamar, it does not seem prudent considering his demands. He is looking for a fully guaranteed deal the magnitude of Deshaun Watson's and that's assuming he hits the open market. It would be hard to believe the Ravens would just let him walk without franchising him a year or two first or getting something from it. Maybe a team like the Falcons who have plenty of Cap space also could offer something.
Justin's trade value has improved, but I'm not sure it is as high as some Bears fans perceive it to be. I dont think we would get a boatload of picks. More like 1st and a 2nd/3rd. He has to prove he can be a consistent passer, and while I think he has what it takes to become that he has to show it.
So all in all, at the end of the day if in some parallel universe we did acquire Lamar and trade Justin, we likely wouldn't end up with many net draft picks and we would have a ton on money strapped to Lamar. Personally I like rolling with Fields and spending Cao space on a good supporting cast