MLB rosters change, through injuries or failure. The trick is to have players that can come up and fill in well as these things happen.
That is the greatest value of a farm system like the Cardinals. No one thought that Carpenter, Adams, Craig, Wong or many others could fill in adequately for Pujols, Holliday, Carpenter, Wainright, etc, and it didn't seem likely that they could force their way into the team, let alone the starting lineup. But as things changed at the MLB level, there were prospects that could step up into the opportunity created.