I don't think the DTs were bad against the run. The problem was they were crap at pass rushing so they could be blocked 1 on 1 which let teams double the ends at times which led to positive off tackle runs. Better penetrating DTs would be great, especially against the pass.
Too much focus on penetrating DL was what started the problems defending the run.
Offenses starting with the Colts in 2018 then the Rams in the playoffs and multiple teams in 2019 tailored their running schemes to take advantage of the Cowboys defensive scheme.
It was an easy "algorithm". If this -> then this. Zone Blocking teams could really take advantage of Marinelli's scheme. Those offenses would run outside zone which has the option for a cut-back to the inside. The RB would "read" the 1tech DT and the 2 LBs (2 LBs in Nickel) and cut back inside or stay to the outside based on how much those players moved to the outside. OLinemen would kick-out on delayed blocks against the LBs to seal them either to the inside or to the outside depending on how far the players moved outside.
The RB really just had to read those blocks. If the blocker is inside of the LB, then that LB is vulnerable to the inside run. If that LB "stays at home", then the OL will kick out and get between him and the outside.
The RBs on those outside zone runs had those 3 defenders to "read" (1tech and 2 LBs).
If the RB just focused on taking advantage of the movement by the 1tech, then he would normally gain at least a couple of yards. If either of the LBs had the same reaction as the 1tech, then the chance of a significant gain is very good. If all 3 defenders reacted the same (i.e. Inside or Outside), then it was almost guaranteed to be a big gain for the RB.
A big part of that was the fact that the scheme was predictable.
Kris Richard got more involved and tried to put band-aids on the run defense by having the 1tech and LBs slow down their reactions to the outside zone run.
- The result was that instead of getting gashed by the inside cut-back like in the Rams playoff game, teams like the Vikings gashed them on the outside because the 3 key defenders had slowed down their reactions to move outside.