Sean Payton was also demoted as an OC with the Giants. Nick Saban had a lot of trouble coaching in the NFL.
It is a complicated profession. One wrong hire, one wrong signing, one run-in with incompetent/ego-driven management or ownership can put a massive dent in a coaching career. DA of course has agency in all this, but that's also why the sweeping generalizations about who he is or these assertions that he's suddenly a dumb-dumb are just ridiculous. He's had success in the NFL. Maybe not as an HC, but he has had success. He may not be right for the Saints, but it's totally plausible that he ends up having success somewhere else.
Or, better yet, in New Orleans, starting with these last few games. That's still the best-case scenario.
Also, worth noting: All of us, including myself, underestimated the importance of Brees and Payton to the success of this franchise.