Led by Pep Guardiola and inspired by Lionel Messi, this Barcelona team could have gone down in football history as the best club team ever, unequivocally and definitively surpassing Real Madrid's stars from the 1950s, Johan Cruyff's Ajax, AC Milan coached by Arrigo Sacchi, Bayern Munich's triple European champions in the 1970s and perhaps a few others.
But now we might never know. Or, at least, we may never be 100 percent sure. By deserting Barcelona, Guardiola is robbing himself and his players of a chance for football immortality.
Barcelona's most successful manager leaves an artwork more than half complete, already beautiful, a masterpiece in the making even, but not all that it could have been had Guardiola persevered for a few more seasons and finished the job.
History's judgment will be this: Guardiola produced one of football's best teams, but perhaps not its greatest, because he did not stick around quite long enough to end that debate once and for all.
In short, he is leaving too early.
The reasons — burnout, dwindling motivation, the fact that there are many rewarding things for a 41-year-old to do in life other than obsess 24/7 about football — are understandable.
Guardiola is a deep thinker, an indefatigable worker and a sensitive soul. It's easy and not very pleasant to see how the burden of leading a club so successful and so demanding of success has worn him down, thinned out his hair and added to the gray flecks in his designer stubble.
Guardiola's consistent refusal to commit long-term to Barcelona, opting instead for one-year contract extensions, always suggested that the day he walked away would come sooner rather than later.
If Alex Ferguson is the bramble of football, a hardy and unshakable thorn in everyone's side for a quarter-century at Manchester United, then Guardiola has been the brilliant flower who drew admiring gasps before withering, exhausted from creating such beauty.
Managers who jump instead of being pushed aren't the norm. More common are those axed when their teams underperform. So Guardiola looked enigmatic for turning his back on success. But human, too, for acknowledging that the job has taken its toll.
Some credit for that should go to Jose Mourinho, Real Madrid's manager who has crafted such a stern challenge to Barcelona this year and now seems certain to wrest away the Spanish league crown.
"Four years are long and they wear you down," Guardiola told a news conference on Friday, which his glum-looking players — but not Messi — attended. "I need to rest and move away."
Still, Guardiola left us that most annoying of questions — "what if?"
If Guardiola had Ferguson's staying power, what further wonders might Barcelona have performed? The mind boggles.
So, a crying shame for football. But wonderful for Barca's rivals. Because even with the genius Messi, midfielders Xavi Hernandez and Andres Iniesta supplying passes and brains and captain Carles Puyol electrifying everyone with his warrior mentality, Barcelona won't be the same — cannot be the same — without Guardiola.
Thirteen trophies — 14 if Barcelona wins the Copa del Rey final against Athletic Bilbao on May 25 — since the former ball boy turned player and club captain took over as Barcelona manager on June 17, 2008, with words that now seem ludicrous: "I can't promise you silverware."
Make no mistake, this isn't just a mere change in personnel, it is the end of a truly remarkable and historic era. Guardiola has achieved more in four years than many coaches do a lifetime. He guaranteed that his team should always be mentioned in any discussion of football's greatest sides.
The Barca style of play — quick passing, emphasis on attack and keeping hold of the ball, teamwork and intelligence — is woven into the club philosophy, taught to youngsters in its universally envied La Masia academy, and will continue without Guardiola. The club and its supporters, the "socios," would likely settle for nothing less.
Nevertheless, it is naive to think that Barca's new manager, Guardiola's assistant Tito Vilanova, will simply be able to pick up next season where his predecessor left off, without so much as a blip.
Perhaps the biggest question is how Messi will adapt to the vacuum that Guardiola's departure creates. Part of Guardiola's genius was building his team around the world's best player. Under Guardiola, Messi evolved from a young star bursting with promise and talent into a three-time world player of the year.
Messi scored 38 goals for Barca in Guardiola's astounding first season, when Barcelona became the first Spanish team to win Europe's top club competition together with the domestic league and cup. This season, Messi has upped his production to an otherwordly 63 goals — Europe's highest total in 39 years.
With Real Madrid set to win the Spanish league and Barcelona losing this week to Chelsea in the Champions League semifinal, some might argue that his team has peaked and that Guardiola is thus leaving at the right time.
Chelsea, like Inter Milan in 2010, delivered a backhanded compliment to Guardiola by playing grindingly defensive football. That has been one of the few ways to withstand the relentlessly attacking football he has demanded.
So Guardiola's departure can only be sad for Barca and for all lovers of entertaining football.
He led a great team to great things. But there will always be the suspicion that Barcelona could have become the undisputed greatest had he stayed.
John Leicester is an international sports columnist for The Associated Press. Write to him at jleicester(at)ap.org or follow him at http://twitter.com/johnleicester