When had Keenum/Murray/McKinnon (with starting QBs and RB on IR), we controlled the clock on offense, extended drives to win Time of Possession battle in 2017. That helped the defense came out fresh everytime and had #1 ranking in free metrics.
Since 2018, with Kirk/Cook/Murray or Mattison, our offense has played for lesser time on the field than defense. When defense stays on the field more and have to come on again after repeated 3-and-outs, of course there will be a regression in their performance. It's not scapegoating, that's how Zimmer wants him offense to keep the defense fresh, but that's not how it is working because Kirk doesn't understand the importance of extending drives, getting to field goal range often and upticking the scoreboard often to put pressure on opponents.
I think you only look at QB rating, TD:INT and personal stats, but I think team work is much more than that. And QBs get paid enormous amount to shoulder that burden and perform. Keenum didn't have great personalities stats (though 23:7 TD: INT is good enough), but he understood the value of extending drives and scoring points against all opponents.
Only games Keenum played very badly was his first game against Steelers as a backup coming in for the team, and the last NFCCG game against Eagles. There was consistency for 15 games in between.
If you have seen my list above, Kirk plays hot and cold, mostly cold when it matters against stronger opponents, at least 5 games in a year.