I don't think it hurts to take an interview. I've had calls from companies wanting to offer me jobs, went for the interview and after a couple of days contemplating a move felt it was in both of our best interests to pass.
Now I don't know about Johnson's case in particular, maybe he decided that being a head coach in Washington put him under too much of a microscope at 37, 38 years old. Maybe after Detroit's run this season he felt that maybe one more year here would be moe beneficial to his career and his family's stability that keeping them in a stable environment in Detroit rather moving them to Northern Virginia or the Maryland suburbs.
He's young, there will be other options if the Lions continue on their current tract.