A few things:
1) if you're throwing in RoMc, might as well throw in Hardy too. Doesn't work that way.
2) ok, so maybe the defense at the moment is slightly better or at least as good as last year's at the end of the season. You're comparing a roster before actual game play and injuries to a roster that lost a lot of players. Hypothetically, we had the best LBers in the NFL last year. They never played in the field at the same time and had a combined 28 missed games including playoffs.
3) our run game and ability to control the clock made the defense appear better than it actually was. Even a horrible defense will have better stats if the opposing team has 20 fewer snaps a game. We don't know if we'll have the same to type of running, so that hurts the defense significantly.
4) despite all these things, our defense was still bad last year and played a big part in us not winning a Super Bowl. You can't trot out a defense of the same caliber and expect to compete for a championship.