Disagree w/ the importance you're placing on corners. Corners have been replaced w/ regularity w/ little dropoff. As long as you have decent backups. It's the trenches that are the hardest to replace, prolly on both sides of the ball. Aside from QB, of course.
The results of replacing any position on the field quite often comes down to the quality of the backup. I'd say mostly.
What you're missing here is that Anderson had already proven to be a quality NFL starter. Yes, he seemingly had motivational problems or something, but the fact remains that he was a quality starter.
You can take any position and look at the best players in the NFL at that position and point to failures. The best CB in the game for a long time was Revis, and he consistently failed. Had to join a super bowl ready team to finally win it. And they couldn't wait to dump him afterwards. Gilmore couldn't get anywhere until he joined a super bowl ready team. Joe Haden, Asomugha, Grimes, Verner, Flowers, the list is long of top corners who didn't get there.
And, once again, you can do that w/ any position. The only position that affects the team enough to drag them to the playoffs is QB. All other positions depend completely on the team.