I totally agree that people all too often want to get rid of coaches without considering the alternatives. I'm not sure if I agree that Tomlin couldn't be replaced with someone better, however. Most of his success came when we had veteran all-time Steeler greats policing the team; we've seemed to lack direction leadership ever since a few of them left.
Also, I know people don't like this argument, but his situation really was unique. Most new coaches are inheriting a bad situation, whereas Tomlin was more or less just passed a torch. And having Roethlisberger in an era where there were only about ten quarterbacks in the world who had any chance of winning a championship (I wouldn't say that is still the case; the league has changed a bit) was a huge advantage.
His clock management and lack of challenge prowess are concerning. He's also a defensive coach who deferred to the system in place when we got here; he didn't really bring anything to the table from an x's and o's standpoint. And, by all accounts, he took the reigns from Butler during that Jacksonville playoff game - not exactly something you'd want to put on your resume.
I like the guy, and I've defended him for most of his career. But I truly believe this team should have won much more than it has. Had Tomlin taken a new coaching position where Rex Grossman was the quarterback and the team was coming off three consecutive seasons of 6-10, 5-11,and 4-12 - a situation more similar to what most new coaches inherit - I doubt any of us would even remember him by now.