I mean.....it was just last season the Cowboys had a good team as capable as anyone. McCarthy as HC and Dan Quinn as DC was working. All they really lacked was Dak's head not going up his own butt at the wrong time. Dak is Quinn Ewers. When he's on, he's on. When he's not, it gets needlessly difficult.
But about Dan Quinn, I think he was the main missing ingredient this year. The defensive personnel is fine, but they clearly missed him in the driver's seat of the D. And Dak being injured much of the season.
I don't think things have been exactly doomsday material for the Cowboys. It's not like they're the Raiders. They needed more reliability from the DC, and they needed a premier running back this year. That's not exactly light years away from excellence.
That said, I don't know why McCarthy left, but I suspect it was a mistake and a net negative.