This is wrong - this is all built around if Dak has another mediocre season, the same with Garrett. That's not "no matter what" - If Garrett say takes us to the Championship and Dak improves upon his play and looks like a top 10 QB or that he could be in the near future, I believe both would have a reasonable argument to be extended.
However, if Dak looks mediocre yet again - that's 3 out of his 4 seasons (which would be 3 in a row), where he hasn't played above being a middling QB.
Like I said, this situation is different - it's not like the HC would be stuck with Dak. There would be no contract, and there would be choices within the draft to move on.
Though, the excuses for Dak are a little more understandable - I don't get the excuses for Garrett being made. Being here over a decade and 9 seasons as your HC, enough would be enough.