Javon Hargrave became a dominant player after leaving. He passed on potential nose tackles including Grady Jarret multiple times (drafted 2015 in the 5th round), Jarren Reed in 2016 (the Artie Burns/Sean Davis draft), Dexter Lawrence in 2019 (taking 220 lb. Devin Bush instead), undrafted Michael Pierce, and Harrison Phillips (2018 draft where Steelers passed on him three times to take Terrell Edmunds, James Washington, and Chukwuma Okorafor).
The 2018 draft is a huge part of the difference between the Steelers and the Ravens right now. As noted, the first three rounds for the Steelers resulted in Edmunds, Washington and Okorafor. Not. Good. The Ravens in the first three rounds drafted Hayden Hurst, Lamar Jackson, Orland Brown, Jr. and Mark Andrews. Yikes.
Tomlin openly admits he does not value the NT position. This is from 2016:
Mediocre Mike Does Not Value NT
Therefore, your entire premise - that Mediocre Mike simply did not have a chance to get a quality NT - is false and disproved by his negative comments about the position and the team's repeated failures to go after a quality NT in the draft, where in keeping with Mediocre Mike's philosophy, they instead select undersized players who turn out to be garbage.