I read that Coto had plans to continue the mirror universe story in the 5th season. They found out that they were canceled during the filming of the second to last episode and had rewrite some of it.

As for the last episode Bevis and Butthead (Berman and Braga) only know how to F'up start Trek. The only season to be remotely any good was this one and they weren't involved until the final episode.