I think there are many truths here, but I think Parcells likes to build things up. He doesn't like to just stick around and ride it out. The challenge for him is taking something that's dreadful and then walking away to the next project, for better or for worse.
He left the Cowboys with a pretty good foundation of building blocks when he left (albeit a questionable transition to the 3-4 defense). He had no control over the next staff to be hired. Unfortunately, Jerry & Co. whiffed on Wade Phillips, but were fortunate enough to find someone with the stomach to build and hopefully stick around and ride it out in Jason Garrett.
I know people hate to hear it, but Jason Garrett's 8-8 seasons are not indicative of what he's been trying to accomplish. The social media, millenial, "instagram" society we live in now is all about instant gratification. I prefer the Garrett model of building a culture and organization that can endure and compete perennially rather than be a flash in the pan and gone from existence like the Arizona Cardinals of a few years ago or any other one-and-done hot team in the past.
All of the Super Bowl champions recently have built up their organizations on certain principles or beliefs and then got the right guy in charge to get the employees to buy in. Seattle, New England, New York, New Orleans, Pittsburgh, Indianapolis. All teams with one voice from the front office to the coach to the players. We've finally got the right combination of that, and ALL of those teams took a while to get where they are. Don't pretend that Sean Payton got to New Orleans or Pete Carroll got to Seattle and it was all 12-4 and deep playoff runs from day one.