Mike Tomlin shouldn't be fired. Starting over will squander Roethlisberger's final years and Bell and Brown's prime. For better or worse, Pittsburgh is stuck with him. He gives them the best chance of winning with this existing team. So no, he shouldn't be fired.
The problem is, he should've never been hired. He is thoroughly mediocre and inflicts wounds on his own team. So while he may give the Steelers the best chance in the now, it's also more likely than not that he will not win another Super Bowl with Roethlisberger. Which really means, he won't win a Super Bowl with the team he built, rather than the team Cowher gave him. Which means, since Pittsburgh did hire him, and will never fire him, even after Roethlisberger retires, this team will just slip into consistent mediocrity (which it borders on, at times, already with a great team...so what does that say about Tomlin's quality and how he will be without a top 3 QB?). And because Tomlin is mediocre and the Rooneys will never sack him...Pittsburgh is going to be in a Marvin Lewis situation in 3 to 5 years; an undisciplined, underperforming team that cannot win a playoff game to save its life but with a coach who will never be fired.