The problem is that Tomlin's failures are largely based on perception, and not reflected in the results to date under his tenure. If the Steelers do go 7-9 this year, it'll be the first season below .500 under Tomlin's watch. Tomlin will still have an overall winning record in the regular season, and would still be one of a relative few active head coaches that can claim to have won a Super Bowl. I think we all see that the line is trending down right now (obviously), but the Rooneys traditionally haven't been disposed to react quickly to these kinds of things, and I think being their "Rooney Rule" hire will give Tomlin some added insurance. I will not be surprised at all (disappointed, maybe, but not surprised) if they extend his contract after this season, in keeping with their usual pattern of extending a coach with one year left. At worst, if the Rooneys really don't have a lot of confidence in him, the extension will be a short one. If they offer him one additional year, that might be the signal that he has limited time to turn things around. But if they extend him for a more typical 3-4 years (I think that's typically been how they've done it over the years), he's almost guaranteed to stick around for the duration and probably get extended again several years on if the decline doesn't continue.
You know and I know that he makes horrible in-game decisions, poorly manages the clock, doesn't make adjustments well, and hasn't been able to keep the team in line in terms of discipline. Now weigh that against the fact that he's 78-46 (.629) in the regular season, and 5-3 in the post-season with a SB win. Coming into this season he was ranked in the top 10 on the coaches' power ranking. And finally, seriously ask yourself who would be available to replace him and be expected to do better? In a vacuum, where you don't consider lots of inter-related issues or worry about realistic solutions moving forward, yes, a 7-9 season should absolutely demonstrate that Tomlin is sliding the wrong way and maybe it's time to part ways. I think the Rooneys take a bigger-picture view, though, so I don't see Tomlin going anywhere.