As a wise man once said (Well
@bkight13 said a few minutes ago), NNNNNNNNNOOOOOOOO!!!
When the Cowboys played the 3-4 there were constant threads of people wanting to switch back to the 4-3.
Parcells claimed finding 3-4 OLBs was easier, but in reality that turned out to be false.
A 3-4 OLB, especially the Strong Side OLB, has to be a 3-tool players. He must be good against the run, good in coverage and good as a pass rusher. Anthony Spencer turned out to be that guy. Fans didn't like him because he was not a top pass rusher, but they didn't really understand the Strong Side OLB vs Weak Side OLB in those defenses. The Cowboys couldn't find anyone else to do that job and ended up Franchising Spencer 2 times consecutively. They had a guy, Victor Butler, that was a decent pass rusher, but he couldn't handle replacing Spencer because he was far interior in pass coverage and run defense and would get confused in regards to when he need to cover, rush or read-and-react against the run.
Finding a guy that can cover and play the run, especially on the strong side is almost mutually exclusive.
Marinelli's defense is a great compromise (At least in theory) between a traditional 4-3 and the 3-4. He shifts the line to the right which puts the SDE head up on the OT but moves the WDE out away from the OT more like a 3-4 OLB. This allows for smaller DEs on the right side. It makes the RDE and the 3-tech DT the top pass rushers instead of both DEs. The SDE is little more like a 3-4 DE; whereas, the WDE is more like a 3-4 OLB. The big difference is that the DEs don't have to play coverage. It's much easier to find a SDE that can play the run than to find a 3-4 SLB that can play the run and play in coverage.
The Cowboys got 6 sacks from Mayowa (6-3, 240) but he likely wouldn't play in the Giants version of the 4-3 when their DEs and DTs are more evenly balanced in their alignments relative to the Center. They need bigger DEs on the right side than the Cowboys need on the right side.
In addition to all of the above, I hate the concept of switching schemes. You've spent years teaching players one scheme and trying to acquire players for that scheme. Now you're throwing all of that away and starting over.
I will concede that Watt
might be that Anthony Spencer type 3-4 OLB that is a 3-tool player, but he was a 1st round pick so it's not as if he was easy to acquire.
Another issue with the 3-4 is that on obvious passing downs, they play a 4-man line like a 4-3 defense except their in Nickel which just like a 4-3 in Nickel is technically a 4-2-5 defense. That means player have to basically learn two positions and it means you carry an NT and backup NT that never play in the Nickel defense.
The 3-4 alignment is a run defense alignment. It does allow for some deception in terms of which OLB rushes, but as mentioned before, 3-4 teams don't use a 3-4 in obvious passing situations.
The ILBs in a 3-4 need to be bigger because they have to take on more blocks from OLinemen. That means they are on average not as good in coverage as 4-3 LBs.
Either scheme can work, but it is FALSE to say that it's easier to find players for the 3-4.
I would like for Marinelli to mix in some 3-4 concepts on occasion which they did a little with Demontre Moore playing as a stand-up DE on several snaps in the HOF game.
Other things you can do to "trick-up" a 4-3 is to get a pass-rushing SLB. The Falcons do it with Vic Beasley (15.5 sacks in 2016). The Broncos did it originally with Von Miller. The Seahawks did it with Bruce Irving who is now in that role with the Raiders. One of the LBs leaves the field in the Nickel so it's a natural fit for that pass-rushing LB to move to DE in the Nickel.
Jaylon would be this player is he gets back to 100% of his college ability.
Marinelli has also "tricked up" things using the 3-man Line. They can go with either 2 or 3 LBs in that alignment. Last year they often used extra Safeties when they playdc a 3-man line and they had good success with it. Marinelli is more likely to "biltz" from this alignment because he can send a LB or DB and still only have 4 players rushing the passer. This allows for disguising where the pressure is coming from similar to a 3-4, but you don't have the issues with personnel like I've explained above about the 3-4. The Marinelli 3-man line is an inverted 3-4. Instead of the LBs being on the outside with the DL close together on the inside, the outside players are DEs and the interior gaps are covered by LBs with a DL in the middle.
The problem with the Cowboys defense has not been the scheme. The problem has been that much more resources have been invested in the Offense and some of the resources that were invested in the defense have not panned out (Randy Gregory, Claiborne, 50M contract for Carr).
