Coaches almost always get extensions. Because if a coach is on his last year with no extension, it's a cloud over the team. Media will ask constantly and if players think the coach may be gone, then he doesn't have the authority he needs in the locker room.
In other words, it's a huge distraction and the steelers don't need anymore of those.
Coaches don't count against the cap. Coaches get fired all the time after signing extensions. All it costs the team is money if they fire a coach with years left on his deal.
In other words, if Tomlin signs an extension and the team goes south, this extension won't stop him from being fired. Coach contracts are usually guaranteed unless they take another coaching job. That's why so many coaches do TV for a year or so after being fired. It's because they are still being paid by their old team while picking up a second TV check.
I'd guess Tomlin would get immediate job offers. He'd definitely get TV offers because of his personality and his credentials. I'd bet he'd be offered the Monday Night gig right off the bat.
So yes, even if you aren't a Tomlin fan, you should want him to get an extension. There is no football downside to it. The only downside could be the Rooneys on the hook to pay him if he was fired and since that doesn't impact the cap, it shoudn't impact the team.