The defence failed because of its construction. While there were boneheaded moves by DQ, and I'm not necessarily advocating for his return as I'd like Vrabel as our DC/Assistant HC (that way, if we do lose McCarthy, we'd have the next HC on the roster). However, as I dissect the Cowboys defence, I think you'll see that the construction of the defence leads to our failures in that department. I think, with this, you will see that our defence was doomed to fail at some point this season. I will then attempt to propose solutions.
The first and biggest area of weakness that I'd like to point out is at linebacker. We don't have any, at least not when LVE goes out with his injuries. Parsons is used too much at DE, and we have safeties playing linebacker. Part of this is on DQ. He likes to have speedy pass rushers, which is great. However, you need physicality at the point of attack. Then again, another problem is due to the construction of the team with us playing a 4-3 nickel style of defence. This doesn't work. It relies way too much on speed, and no where near enough on physicality. It's why, when you see us play QBs who are statues in the pocket, they feast on them. If we play a mobile QB, we have a problem. Additionally, this is why we can't stop the run against physical teams like the Packers, 49ers, and Bills. As we have seen, if you can run the ball effectively, you can torch our defence. If your only really good at throwing the ball, this team will feast on you.
The second area of weakness is where we play Micah Parsons, in part because of how the linebackers are constructed. Micah is in the 250lb range. Most offensive linemen are around 300+. That's a significant weight difference. Yes, he's strong. However, using him as a 4-3 DE or sometimes even DT when he's rushing the A gap means he's having to take on the hardest of the blocks usually to get to the QB. He does well at this early in the season, but he's clearly gassed by the end of the season. Then, come playoff time, he's useless. Plus, he's in a spot where he can't really get to the RBs as effectively as I'm sure he'd want to, and shifting him takes him away from what he does best (which is rushing the QB). I think there is a better way to use him, which I will state when I discuss solutions.
Our other issue is depth on defence. Part of the reason that we have safeties playing linebacker is because we had about 3 linebackers on the roster, plus Parsons who doubles as a DE. Part of the problem is that Jerry doesn't really do much in FA. By relying on the draft and bargain brand Free Agency, it puts you behind the 8 ball when injuries happen.
Another problem stems from the offense. The running game problems is another reason the defence failed. Using Tony Pollard like he's Zeke was never going to work. Mike McCarthy made it crystal clear that he wanted to run the ball. The FO thought that Pollard could be used like Zeke. Surprise, he can't. When this happens, the defence will be on the field more.
As you can see, the way the defence is constructed leads them to failure later in the season. In football, you have injuries. As that happens, you must have depth to withstand the problem. So, here's my attempt at solving this problem.
Firstly, I'd focus this upcoming draft on 3 places. Linebacker, running back, and NT. I'd take at least two linebackers this draft. If Penix doesn't drop to us at 24, take the best linebacker available, and then take another in round 5 or 6. If Penix does (assuming we go QB round 1), then go LB in rounds 2 and 5 and/or 6. I'd then go running back. I'd like the back from Texas (Brooks) in round 3. He's an up the middle back. I'd then use him there rotating occasionally with Dowdle and allowing Vaughn to be the scatback option. This should help us to control the ball and the clock more. It will make us less dependent on playing with a lead considering that our defence won't be on the field nearly as long. I'd then try to shift our defence to more of a 3-4 base. I know 4-3 is a traditional Cowboys bass, but I think the most effective way to use Parsons is our of a 3-4 base. This gives him some better protection because the DEs and DTs in a 3-4 have to be more physical. It's more predicated on stuffing the middle and allowing the LOLBs and ROLBs to rush more freely. Plus, Parsons can then rotate to different spots at LB to rush, depending on the play. Parsons can rush from the outside, or can blitz from a MLB spot as the 3-4 would open up some spots for him to rush. Plus, with Overshown coming back next year, he should be able to rotate with him on the outside for max effect.
I'd focus the defence more on physicality and less on the speed that our DEs have. The DBs still should be ballhawks, but we can't just rely on getting takeaways. We do want takeaways, but we should be constructing the team to be more physical. If we can become more physical and effective at stopping the run, the takeaways will happen. But, the 3-4 and bringing in more physical linebackers would allow us to stop the run more effectively, and it should result in better pass rush options.
I'd also get another DT this year, but this one Must be a NT. Mazi and Hankins would be 3-4 DEs in my view, and we would need a proper NT to clog the middle. Finally, I'd use FA to try to provide more depth at linebacker and NT.
If we do this, I think our defence can be more effective. Parsons won't be exhausted later in the season, and the team can be more effective.