They will be remembered as the greatest team of their generation, a soccer superpower unrivaled in the 21st century, established by Xavi and Ronaldinho and Samuel Eto’o, propelled by Carles Puyol and Andres Iniesta and Gerard Pique, supported by Thierry Henry and David Villa and Cesc Fabregas, sustained by Dani Alves and Javier Mascherano and Sergio Busquets, prolonged by Neymar and Luis Suarez, and of course immortalized by Lionel Messi.

They are Barcelona, winners of 30 trophies over the past 13 seasons, still one of the five best teams in the world, forever and always a delight.

And yet, suddenly, as the most monumental transfer saga ever has completed, they look vulnerable.

They are no longer the Barcelona of 2006, nor the Barcelona of 2009, 2011 or 2015. They are, in the blink of an eye, a team coming off a second-place La Liga finish and a Champions League quarterfinal exit, their six most influential players the wrong side of 29 years old, their manager new for the fourth time in six years, and their cupboard, while far from bare, not exactly teeming with world-class talent. Messi and Suarez are both 30. So is Pique. Iniesta is 33. Busquets is 29. So is Ivan Rakitic.

Neymar is 25. Neymar was the future. And he still is. But he is no longer Barcelona’s future; he will soon be PSG’s. And now Barcelona’s is as murky as it has been since 2002-03, two seasons before Messi made his Camp Nou bow.

There is no need to panic, no need to schedule a funeral for this extended era of greatness. This is not an overreaction to a mildly underwhelming season. Barcelona as we know them are still alive, and they are still great. They could still be great for years to come. They could.

But for the first time since Messi’s 16th birthday, there are fissures. There are real, pressing questions. There are unknowns. Why?

Because the Catalonian club, in recent years, has failed to replenish its squad as it did so expertly for a decade.

Throughout that decade, roughly from 2005 through 2014, Barcelona kept hiccups to a minimum because it constantly bought and developed its next wave of players. It was part shrewd business, part wonderfully coincidental timing.

When Ronaldinho and Deco declined, there were Iniesta, Xavi and Messi ready to step in and steer the club to glory. Building around those three, Barcelona always had replacements at the ready. There were rarely interruptions in the lineage, and when there were, they were often filled by youngsters or accounted for by managers.

But ever since Suarez and Rakitic spearheaded the latest retooling effort in 2014, cracks have started to appear. That summer of 2014 also brought some short-sighted signings. The following year brought the transfer ban, with 28-year-old Arda Turan for 34 million euros the only signing of note. The ban’s legacy still lives on in the blatant gap in the current squad’s age profile between 25 and 28.

The verdict on last summer is still pending, and offers hints of hope. Samuel Umtiti should be a regular at center back for the foreseeable future, and Lucas Digne could be the Jordi Alba heir apparent. Both are under 25.

But 2016’s banner signing, Andre Gomes for 35 million euros, was a mistake the minute it entered the mind of anybody at the club. Paco Alcacer, the second-most expensive of the lot, has a ways to go to reach the level Barcelona requires of a first-choice forward. Given the total expenditure of 122 million euros on six players, the early returns aren’t great.

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Meanwhile, the club’s famed academy isn’t churning out stars at the rate it used to. Spain’s under-21 squad that reached the final of the UEFA championship this past summer featured zero players developed and kept by Barcelona past their 21st birthday. In the current squad, the only academy product under 29 years old whom Transfermarkt values above 15 million euros is Sergi Roberto. Other prospects have been set back by injuries (Rafinha) or simply haven’t progressed.

Spain’s second-most productive academy, Real Madrid’s, has yielded current first-teamers (Dani Carvajal), promising youngsters (Marcos Llorente, Borja Mayoral) and hefty transfer fees (Alvaro Morata, Jese Rodriguez). Los Blancos have also brought in teenagers from around the country (Marco Asensio) and abroad (Raphael Varane) to supplement it. All the while, Barcelona’s La Masia hasn’t borne as much fruit.

All of this left Barcelona susceptible to a day like Thursday, when its only prized asset with more than half a career ahead of him effectively bolted for Paris and left it staring at uncertainty.

The situation is by no means dire. With Messi as good as ever, Barcelona will be able to cope just fine for now. And the 222 million euros recouped in the Neymar deal will certainly help fund the next round of squad replenishment.

But the club has to get that next round right, just as it has so often this century. If it can’t, Neymar’s move could be the leak that starts the flood. And the flood could wreak more damage than many of us realize, enough to incite a downswing, the likes of which Barcelona hasn’t experienced in 15 years.

More soccer coverage from FC Yahoo:
• Neymar clear to join PSG after representatives terminate Barca deal
• La Liga refuses PSG’s buyout payment to Barcelona for Neymar
• Why Neymar is actually worth his astronomical sum of money