Realistically, I agree the city will do everything it can to continue using the stadium. My suspicion is that Chicago will do something like add a roof to Soldier Field in an effort to court NFL teams looking to make a move or stump for an expansion team, but time will tell.
It's worth pointing out, though, that those three stadiums will be open by the time the Bears move, with modern amenities and also better suited to specific events, be it soccer games or college bowl games. Soldier Field is a neat venue, but we also know it's not without issues.