Well, how are Dat and Bradie a comparison? Last time I checked, they play different positions. However, I think I see what you are getting at. A player who stepped up in an injured vet's stead. Shanle might have been more appropriate.
James, although he has played better this year, was still involved in the gaffe at the end of the Giants game that almost cost Dallas a win. I seriously doubt Dat misses getting that coverage call sorted out. Shanle's big plays in the nickel have dropped off since he is playing full time as well.
My point, I guess, is that I think the defense has improved due to the familiarity with the defense and the abundance of nickel being played. I think it would have been even better had it not lost Dat. While James has proved to be serviceable as opposed to being a liability like last year, the presence of Nguyen will help the defense on his return. He was consistently making plays.
So, back to the original thought. Tucker is out and people are spinning it as positive, which I don't understand. Your point, I believe, is that Tucker may indeed step up and be serviceable. We may not see the offense miss a beat similar to the way we have seen James step up in the 3-4 (or Shanle in Nguyen's absence). I sincerely hope that is the case and my concern is unfounded.
That doesn't diminish the fact that the team is losing a starting left tackle who has been to the Pro Bowl. Inconsistent as his play may be, he was still the best LT on the roster or he wouldn't have been starting. So, now your depth becomes your starter and your depth disappears. Add into the mix a QB who is only as effective as his pass blocking combined with a guy who was ineffective at RT last year and has no game experience there and you have a recipe for disaster in my book.