Why is Erik Gustafsson Eating Valuable Minutes?

Well, the trade deadline has come and gone and somehow Erik Gustafsson is still eating minutes on the Chicago Blackhawks back line. Whether using the «eye test» or analytics, Gustafsson is simply not a very good defenseman. Why is he stealing minutes from a young defenseman who could be part of the rebuild moving forward?
When Stan Bowman made one of his famous «bring 'em back» to the team moves, fans were confused. At the time, he was claiming rebuild. This was not part of a rebuild move. Of course, we found out that it was more of a retool, and Gustafsson would add much needed scoring, or so Bowman thought.
In the 2018-2019 season, Gus tallied 60 points and he was part of a really good powerplay squad. Those numbers dropped the following season to 29 points, then to 12. This year in 50 games, Gustafsson has only scored 16 points. Combine a low scoring total with suspect defense and the Hawks have a recipe for disaster.
I have to believe that the Blackhawks were willing to move Gustafsson at the trade deadline. He is 30 and an unrestricted free agent at the end of the season. He is surely not part of a rebuild. I assume that the asking price was too high and other teams were not willing to part with much in the way of draft or prospects for a defenseman who isn't offering much on either end of the ice.
So then, if other teams see no value, his defense is lacking, and his scoring touch is lost, why are the Chicago Blackhawks giving him minutes over the youngsters in Rockford, specifically Alex Vlasic.
Vlasic is a very large defenseman, standing 6'6» tall,weighing 200 pounds, and he is only 20 years old. Because of his size, the Blackhawks should be more willing to use him more frequently; he can take the physical toll the NHL game brings. He comes from a strong program at Boston University, so he should be sound in terms of the basics. All he needs is some time on the ice to develop alongside a veteran defenseman. Chicago is in a rebuild, so this is the perfect time. He will get game experience down the stretch and then an offseason to prepare for next season.
Stop giving Vlasic's minutes to Eric Gustafsson.