An excellent bit of sleuthing from Tribune Review reporter Mark Kaboly, who laid out a list of Steelers assistants and coordinators who have left the team for one reason or another since 2010.
Standing out in that is the bit on how Tomlin didn't make a coaching change in his first three seasons with the team. From 2010 through yesterday, the team has parted ways with 14 assistant coaches, creating one of the highest turnover rates in the league.
The most recent, and probably the most noteworthy, is the resignation of Steelers defensive coordinator Dick LeBeau, who, according to several media whispers, was forced out (it seems unlikely the 77-year-old LeBeau would say he's not retiring, he's resigning, if he was expected to return to the Steelers in 2015). Joining the forced-out side are former offensive coordinator Bruce Arians, special teams coaches Bob Ligashesky, Al Everest and Amos Jones and offensive line coaches Larry Zierlein and Jack Bicknell. Add Sean Kugler to the group, but he appeared to leave on his own accord, accepting the head coaching position at his alma mater of the University of Texas-El Paso.
It's instructive to note here too, 2015 appears to be (as of now) the first season in which now fourth year offensive linemen David DeCastro, Kelvin Beachum and Mike Adams will have the same offensive line coach as they had the previous year. It started for them in 2012 with Kugler, went to Bicknell in 2013 and it appears Mike Munchak will be back for a second season.
Former wide receivers coach Scottie Montgomery returned to his alma mater as well, accepting basically the same position at Duke. Ex-Steelers running backs coach Kirby Wilson took a lateral move last season to coach the running backs of the Minnesota Vikings (and was allegedly a candidate for the Jaguars' offensive coordinator job).
Some were promoted as well, with Ray Horton having accepted the defensive coordinator position in Arizona, Lou Spanos becoming the linebackers coach in Washington and Harold Goodwin taking the offensive line coach in Indianapolis.
Ex-quarterbacks coach Ken Anderson retired, completing the full spectrum of direction for former Steelers assistant coaches. Arians probably highlights that group in terms of overall success, as he's a strong candidate for his second Coach of the Year award. But looking back on it, it would appear when Tomlin was given a stronger level of control over his coaching staff. And he's certainly taking advantage of that leverage.