I hear what you're saying, just a few points about drafting a new franchise QB.
Unless you're the Browns, you don't change HCs and/or GM at the start of their careers. A big change can alter their development. Ben at least had 3 years to develop with Cowher before he left and Tomlin took over, but it was Cowher's decision to retire, so I understand what he needed to do for his family.
Now on the other side, if the steelers end up with 4, 5, 6 win seasons over the next 3 years and Kenny is a failure, I do think the Rooneys would look to move on from Tomlin. I know Noll had better records, but he was aging and not relating to the new players. It was time to get on with his life's work, and if we have some bad seasons with KP and a great D already built, it would be time for Tomlin to get on with his life's work as well.