Historians push back on the popular version of this story pretty hard. How much was actually irreplaceable, and how much is myth?
The "single catastrophic destruction" narrative is itself a myth. The library declined gradually over centuries through reduced funding, political disruption, and the shift toward Constantinople. By the time of the various fires people attribute it to, much of the collection had already dispersed or deteriorated. Historians of the ancient world largely agree on this.
Agreed on the gradual decline point. But the question of what was irreplaceable is still interesting. Even if the texts were available in copies elsewhere, Alexandria was a concentration point for translation and commentary - the scholarly infrastructure around the texts. Whether that was recoverable is a different question from whether the texts survived.
The counter-factual problem is enormous here. Even if you could quantify what was lost, claiming it "set humanity back centuries" requires modelling an alternative history of knowledge transmission that nobody can actually construct.