I can assure you this was a resignfired situation.
The Steelers don't fire coaches. This also allows for compensation to the team if any of these teams (like the Giants) want Tomlin to coach next season.
Effectively a mutual parting of ways
The situation has slowly come to this point over the last 5 years.
There is no way Steelers Nation was going to allow yet another year of Tomlin - it had boiled into a toxic situation.
Look, thank you for your service. You won a Super Bowl. You always kept us relevant. But there are a lot of weaknesses under the Tomlin regime and it was time for a change.
He will likely coach again if he wants to. Plenty of terrible teams need a stabilising player-friendly coach to come in and right the ship a bit - Tomlin will get his guys to play hard every game. He's perfect to take a team like the Jets, Giants, etc, etc to some modicum of respectability.
But he ain't getting a good team over the top