For such well coached and prepared teams they sure lost a lot of games. Not having an elite QB doesn't explain that away. I mean DA has never had an elite QB but he seems to be able to hover around .500 and he is far from a good HC.
As long as there is progress a young head coach can survive 3 or 4 years. And some even occasionally catch lightening in a bottle. Look at Payton or Zac Taylor. Or McVay or any number of other successful coaches that got their first job without prior HC experience. It happens all the time in the NFL that a team hires a young head coach who develops a team at a low point for a period of 3 to 4 years. And the guys that do that and are successful are the guys that turn into great coaches.