GM moved out of the old GM Building into the GM RenCen in 1996 when Jack Smith was CEO. It was NOT a mistake moving out of the old GM Building on Second and W. Grand Blvd. It was a tired old building that had all kinds of issues. I remember that GM bought the RenCen for a song, actually less than it would have cost to rehab the old building. When Ford moved out of the RenCen, much of it was unoccupied which is why GM bought it so cheap.
Financially it was a wise decision. One of the big issues at the old GM building was the security for employees. The RenCen is a 'city within a city' with many restaurants and services within the building. It has limited access, and all entrances are guarded. Access to the working areas of the building are by invitation only for non employees. Even employees who don't work there normally are required to check in at the reception area for each tower.
Having said that, I worked at the Tech Center in Warren, MI for several years and had no security issues. It is an open campus (access to each building is limited) with plenty of open parking, and people eat lunch and walk around outdoors whenever they want to.
I don't think it's realistic to think that GM could move all of their headquarters staff outside of the SE Michigan area, just too much expense and disruption. And to move headquarters 17 miles north to the Tech Center would have been a huge issue then and now. I know there was some consideration to do that around the time of the bankruptcy, but they opted not to do it.