The problem with this reasoning is far too often people use it to justify poor defense even in the modern context.
We're not ranked 30th/90th/whatever because the game has changed. We're ranked that because there's 30-90 better defenses.
The only thing stopping Nebraska from being elite on defense over the last 7 seasons has been Nebraska.
Define 'better'. The standard definitions don't work because they don't adjust for pace. Football statistics are awful in this regard. It's a drive based game in which drive based stats are incredibly hard to come by.
I like to use simple differential stats, because at the end of the day it doesn't matter what your offense or defense does, it matters what the difference is between what you generate minus what they generate. That's how it works in an alternate possession game. You can't really do much to change the number of possessions, so you just change how efficient your possessions are relative to the opposition.
Improving on what Diaco did will satisfy most fans in the short term. Chinander has one of the easiest jobs in America.
All Diaco had to do was outperform Banker. He didn't make it through Arkansas St before his leash expired. Chinander faces steep initial criticism. He won't have much of a cushion, and he knows that. I like that about him. He knows the expectations are high and he understands what not meeting them means in this business.