Hard to say if it's Stephen, Jason, or the combination, but it really doesn't matter. Stephen's a huge supporter of Jason Garrett, so if he's the one making the good decisions and Jason is his pick for coach, it stands to reason he's likely the right guy for the job.
Nobody has any fear of dumping Jason Garrett. Most reasonable people don't want to dump Garrett because it would be a stupid thing to do. Consistency isn't that easy to get in the NFL. When you finally establish some, can demonstrate that your talent selection and development teams are doing their jobs, have roster unity and staff unity, and an upward trend with the performance of the team, why in the world would you dismantle that to bring in a new regime?
I'd also reject categorically that the team is losing playoff games because of the head coach. Not too long ago, I was rejecting categorically that we were losing regular season games because of the head coach. That was when the complaint was that Jason couldn't get us better than 8-8. We're losing playoff games because we have insufficient talent on defense. We've got picks, a roster that's clearing of cap space, we're probably the most desirable destination team in the entire league for defensive free agents, we've got a staff that can find and develop talent, and we might end up with a surplus of picks if Tony gets traded in the offseason. All we have to do is stay the course and keep fixing the roster at a faster rate than other teams are fixing theirs, take advantage of two of the next three years of no big QB contract, and contend.
The fact that we have a large number of fans in a Coach of the Year award thread who instead want to fire that coach and start again is mind boggling.