I don't know why but it seems there are always fans ready to pounce on the best players and blame them for all the problems the team has. Trade Dak was big at one point. Trade Diggs is still raging. Trade Micah was running around for a while, until we traded him.
Players come and go. They retire, get hurt, or just quit. New players take their places. The trick for any GM is to continuously upgrade the level of talent on the roster. Its that last part where Jerry seems to fail the most. We lose players and replace them with worse players too often.
Of course Micah made the defense better. Only someone deficient would say otherwise. I watched the Packers-Bengals all-22 today. If you just look at the stat sheet you might conclude Micah didn't do anything. But if you watch the video you will notice that, a) is not terrible against the run, and b) the number of double and triple teams he gets frees up other players to make plays. That's what Micah does. He forces the opposing offense to account for him. You do not see him getting blocked by TEs or WRs. There was one play where the Bengals brought a TE across the field in motion to block Micah and Micah just tossed him aside. Then there was an OT waiting for him. Contrast that to the Panthers using there TEs and WRs to shove the Cowboys DEs out of the holes so Dowdle could run for 7 yards. There is no doubt if Micah was still playing for the Cowboys he would make the defense better. But defenses are still a team effort. Expecting 1 player to change the game is silly.
Dallas has had a problem stopping the run for years. It is not because Micah is not good against the run. It is because their front 7 is not good against the run. The CBs do not support the run, and the safeties are not particularly instinctive. This is why there is no trade Jerry can make at this point to make this defense good.