Idk about that. The truth is he had some offensive talent from 2014-2018 that was as good as anything we have ever seen in Pittsburgh, as good as those great '78 and '79 teams, and he did nothing with it. I object to this idea that Tomlin was at fault for everything under the sun as well, but the truth that he accomplished nothing with those teams will not be easily forgotten.
I hear everything you are saying diver but need to add a few things
1). Tomlin did a lot of things right, he had to in order to achieve that non-losing season status. In doing so, he also lowered the expectations of the Franchise, something that took years of mediocracy to do. Beating NFL teams during the regular is a hell of a lot easier than beating teams in the playoffs.
2) Tom Brady, hated for cheating, being caught cheating on something his own coach call isolated to Tom himself proving he was responsible for the cheating of said deflated-gate activity. He is now remembered as the GOAT and made comparison to in reference to any other good QB reference.
3). The crowd is fickle, they remember nothing.
The overall displeasure comes mostly from his inability to win a single playoff game in 10 years. Not just failure to win, but the fact of being humiliated multiple times. This franchise was accustomed to play off wins and deep play into, something mediocre Mike could not achieve. That has got to be a lot of what the disappointment is seeded in, which many will forget.
If his TV career blooms ( think Warren Sapp / Kurt warner) that will make it even easier for his forgetting of.
Currently he is remembered for his failures to Steeler nation fans, with time that may lower.
Salute the nation