Johnson did not suffer a major concussion last season. Aikman was younger when a concussion ended his career. So was Steve Young. Both guys may have been able to continue playing, but head injuries are nothing to play around with. Green may be two years younger than Johnson, but that head injury may have changed him as a QB.
Do you want Dallas to trade a draft pick to KC and still pick up Green's hefty salary for him to be the back up? That just is not a smart move. If Green had not suffered a concussion, then maybe it would be a smart move. Then again, KC would probably not be looking to unload him if that were the case. It is not as if the Chiefs have a young QB. Huard will be 34 when the season starts. If Green is ultimately released, then Dallas can re-evaluate at that time. Of course that will be telling as well in that no team, even ones looking for a starter would be willing to trade for him.