The offense is almost irrelevant to this discussion.
Rex earned his dismissal because his schemes on defense have not worked, and he hasn't adjusted them to the realities of the modern NFL where you don't get to practice much and you have constant churning of the roster due to injuries and the salary cap. And on top of that, he doesn't have his players prepared enough on gameday. The poor communication we saw last year continues to plague this defense. Guys running on the field late, yelling at each other, lined up wrong, IT'S STILL HAPPENING. That's on Rex and it's inexcusable.