As Don Shula’s coaching career in Miami was winding down in the mid-1990s, I had a running debate with a friend over the coach’s place in NFL history.

I claimed he was the best pro head coach of all time, based on most wins ever, 347, consistent success for 33 seasons in two cities and six Super Bowl appearances, two of which he won.

My friend, a regular critic of Shula, thought I was living in the past. Shula, he pointed out, hadn’t reached a Super Bowl since 1984. He didn’t see how I could rate Shula over such coaches as the 49ers’ Bill Walsh and Redskins’ Joe Gibbs, who didn’t gather much dust between Super Bowls.

Those coaches, I replied, didn’t stay around long enough to hit the skids, as usually happens even to championship coaches. Walsh won three Super Bowls in a decade and Gibbs won three Super Bowls plus an NFC title in 12 years.

Both are Hall of Fame coaches and had to keep oiling and repairing their machines, but they resigned at their peaks. As baseball managers like to say when they remove a pitcher who hasn’t yet run into trouble, Walsh and Gibbs (prior to his unspectacular comeback) came out clean.

Which brings us to recently-departed Broncos coach and football operations chief Mike Shanahan and Chiefs president Carl Peterson. Both have been among the league’s mainstays, ending runs of 14 and 20 years, respectively.

Shanahan’s Broncos earned seven playoff berths and won two Super Bowls. Under Peterson, the Chiefs earned seven playoff spots in the 1990s and advanced to the AFC championship game.

Shanahan was fired after missing the playoffs for three straight years. Peterson oversaw just six wins during the past two years.

Did these guys forget how to win? Certainly, Shanahan had his best success with John Elway at quarterback, and Peterson with Marty Schottenheimer as his head coach.

Yet that overlooks the law of the NFL jungle. The longer you stay, the better your chances of getting devoured. Young, hungry lions are on the prowl every year. The vast majority don’t survive, but the ones who do pose a constant threat to run the old lions out.

Remember, this is a league that tries its best to lift the bottom teams through the draft, free agency, waiver system, salary cap, revenue sharing and, to a diminished degree, the schedule. To keep beating your opponents, you must keep making better decisions than they do, even when the system leaves you picking through their leftovers.

That’s partly why head coaches no longer run the same team for decades. The Titans’ Jeff Fisher, officially hired as head coach in 1995, is now the NFL’s coaching dean after sharing that status with Shanahan. They inherited that title from the Steelers’ Bill Cowher, who retired in 2006, a year after a Super Bowl win. He’s now the coach everybody wants, partly because he left with his reputation unblemished.

NFL coaches and general managers live with a keen understanding that they walk a fine line between success and failure. With a key personnel misstep here and there, a coaching hire that doesn’t work out, excessive injuries, off-the-field issues, or a bad bounce or call in a big game, a once-formidable machine can fall apart. And it can fall apart seemingly overnight.

Shula’s machine never fell apart, yet he stepped down after an embarrassing playoff defeat at Buffalo in 1995. Dolphin fans were ecstatic to welcome Jimmy Johnson, whose Super Bowl magic in Dallas convinced them he would quickly return the Dolphins to glory.

Johnson’s four seasons in Miami, however, didn’t match up to Shula’s last four seasons. Johnson’s regime ended with a 62-7 loss at Jacksonville — a far cry from the two Super Bowl wins that climaxed his brief but glorious stay in Dallas.

Regarding the NFL’s law of diminishing returns, I rest my case.