Carroll was always a good coach. He didn't exactly get a fair shake his last couple of years in Seattle. Wilson couldn't run his system anymore (similar to how Wilson couldn't run Paytons), and Carroll took up Smith as a place holder while he searched. They removed Carroll and nothing in Seattle has changed. Their still semi-competitive, looking for a franchise QB trying to replace serviceable players with younger talent.
The one mistake Carroll made was letting his roster age and taking action a bit too late. But they were also competing for a superbowl in that duration... similar to how Loomis acted like we were constantly 1 player away for several years.