Lotta effective 34 yo WRs. No reason to dump him because Hundley can't find him, he was doing fine with Rodgers but he is not the same player. FA is mixed bag. They are all on market for same reason Nelson would be; no longer the same value or talent. You have to replace the talent AND get him for cheaper to make a switch pay off unless you have a younger player ready to go.
Cobb is slightly different as he might be effectively replaced by Monty. But that requires another position switch.