The Flex defense was great till enough film was on it and enough coaches had moved on from the Cowboys who knew all the keys and that a backside TE would destroy it.
Buddy Ryans 46 was awesome with the right players, till enough film on it
Tampa 2 was excellent until.
and so on and so on and so on.
Mike Zimmer can coach a 4-3, but he doesn't have the right players for it, and even if he did.......the book to beat it is out there.
Dan Quinn has a beautiful D, but the book is out there. San Francisco and Green Bay have used it to perfection.
Either get a young coach that can design newer better stuff,
or get better players that can adapt to differing defensive ideas based upon what the offense can do.
Dallas has neither the coaching, nor the "better than everyone else" players., nor adaptable players.
Dan Quinn's defense was better than a new 4-3 without the players and a full book on it, but it still wasn't going to win the Superbowl.