I've always thought one of the trickiest things in team sports is figuring out how to stay at a championship level after you win a championship.
You can't sit still. You can't keep the same team. You can't just sit around and expect to win another title.
Because of free agency and the salary cap and aging players with declining skill and incoming draft picks you're going to have significant turnover.
If you stand still, you fall behind. So the challenge is retaining what made that team special while turning over a minimum 30 percent of the roster.
And it's really hard to do that.
That said, here are the Eagles 17 months after winning a Super Bowl and there are 28 guys who were on the Super Bowl roster - either on the 53 or Injured Reserve - who are no longer here.
That's a huge chunk of a championship team. More than half of the guys who played in the Super Bowl in Minneapolis are no longer Eagles.
Yet the Eagles are still an elite team, a legit championship contender.
Howie Roseman, the departed Joe Douglas and the scouting department have done a remarkable job replacing the guys the Eagles have lost or moved on from without losing what made that 2017 team so special.
Who's arrived since 2017? Who's left?
Let's take a look at how this process of replacing key parts of the 2017 Super Bowl team has gone:
Lost: LeGarrette Blount, Jay AJayi, Kenjon Barner, Darren Sproles
Gained: Miles Sanders, Jordan Howard
Barner is the only running back from the 2017 team that's even on an NFL roster right now. The Eagles got brilliant play out of Blount and Ajayi but knew when to move on. With Sanders, they finally have a young potential franchise back for the first time since they drafted Shady a decade ago.
Lost: Torrey Smith, Marcus Johnson
Gained: DeSean Jackson, JJ Arcega-Whiteside
One position without a lot of movement. Alshon and Nelly are still here from 2017 but now the Eagles have a third legit receiver to go with them and potentially the best wide receiver lineup in team history. Smith caught only 17 passes for the Panthers a year ago. Mike Wallace was here last year but played in just two games with no catches.
Lost: Nick Foles
Gained: Clayton Thorson, Cody Kessler
Obviously until Carson Wentz wins a Super Bowl there are going to be fans second-guessing the Eagles' decision to cut ties with Foles and sign Wentz to a long-term deal. But the reality is the Eagles lost a Super Bowl MVP at quarterback but still have one of the most talented QBs in the league, a legit MVP contender.
Lost: Brent Celek, Trey Burton
Gained: Dallas Goedert, Richard Rodgers
They moved on from an all-time Eagles great in Celek and a promising young tight end and Philly Special hero in Burton, but they managed to replace them with Goedert, who has the ability to be a top-10 tight end playing alongside Zach Ertz. They cut ties with two popular, productive, capable players but got even better. This is how you remain elite.
Lost: Chance Warmack, Will Beatty
Gained: Andre Dillard, Jordan Mailata, Matt Pryor
The Eagles have managed to keep the entire starting o-line from 2017 intact while adding their left tackle of the future in Dillard and an intriguing young prospect in Mailata.
Lost: Chris Long, Steven Means
Gained: Josh Sweat, Shareef Miller
Derek Barnett was already here in 2017, but he's the key to moving on post Chris Long (and Michael Bennett, who wasn't here in 2017). If he stays healthy, the Eagles will be fine at defensive end.
Lost: Beau Allen, Destiny Vaeao, Elijah Qualls
Gained: Malik Jackson, Hassan Ridgeway, Treyvon Hester
A lot of turnover at defensive tackle, with Tim Jernigan coming and going and Cox the only tackle who's been here straight through since 2017. But definitely a stronger, deeper group than the Super Bowl season.
Lost: Mychal Kendricks, Najee Goode, Dannell Ellerbe, Jordan Hicks, Joe Walker
Gained: Zach Brown, Paul Worrilow, L.J. Fort
Hicks and Kendricks are gone, and both were talented but each had issues. The Eagles brought in a patchwork of guys the last two offseasons, but with Nigel Bradham, Kamu Grugier-Hill and Brown, they should have a capable group.
Lost: Corey Graham, Patrick Robinson, Jaylen Watkins
Gained: Andrew Sendejo, Blake Countess, Avonte Maddox, Cre'Von LeBlanc
Robinson in particular was such a huge part of that 2017 postseason run and Graham played really well in 2017 as well, but the Eagles have put together an auspicious stable of young corners. This is potentially a terrific secondary.
Lost: Donnie Jones, Caleb Sturges, Chris Maragos, Bryan Braman
Added: Jake Elliott replaced Sturges early in 2017, and both he and Cameron Johnston have proven to be a sound kicking combo. Maragos and Braman were both valuable special teamers, but the Eagles have plenty of young linebackers and defensive backs who've been able to replace them.
Click here to download the MyTeams App by NBC Sports! Receive comprehensive coverage of your teams and stream the Flyers, Sixers and Phillies games easily on your device.