Ennis is a craftsman. He found what works and what he enjoys, then stuck to it. Sure, it's artistically bad to stagnate, but he's not a fancy pants bohemian. He's a man who writes a good story (or a cringey one at worst, but still entertaining), submits it to the publisher and gets paid.
You have to respect that, since a lot of people can't seem to achieve it (case in point: most of Big Two's writers).