This dude improved pass rush every year and when he left the room they declined.

Per Grok-

When DeMarcus Covington was the defensive line coach for the New England Patriots from 2020 to 2023, the team showed varying levels of pass rush productivity:

2020: The Patriots ranked 11th in the NFL for pressure rate, indicating a solid performance in generating pressure on opposing quarterbacks.
2021: They improved, ranking 9th in pressure rate.
2022: They again enhanced their performance, ranking 8th in pressure rate.
2023: The Patriots achieved their best ranking under Covington's tenure as defensive line coach, coming in at 3rd for pressure rate.

Regarding improvement, there was a clear upward trend in pass rush productivity from 2020 to 2023. The team consistently improved in generating pressure, with the most notable leap from 8th to 3rd in 2023. This suggests that Covington's coaching had a positive impact on the defensive line's performance in terms of pass rush during his time in that role.

However, in 2024, when Covington was promoted to defensive coordinator, the team's defense struggled, particularly in pass rush effectiveness. The Patriots finished the season ranked 27th in the NFL for sacks, which indicates a significant drop in productivity compared to the previous years under Covington's direct coaching of the defensive line. This downturn could be attributed to various factors, including personnel changes and the broader defensive strategy, not solely on Covington's transition to coordinator.

It certainly wouldn't be the first time a very successful position coach didn't find that success translate when promoted to DC.
Some position coaches don't make good Coordinators, while some Coordinators don't make good HC's. It happens.

I'll take the glass half full approach on this hire based on his 4 years as a DL coach, as opposed to his 1 year as a DC.