Here's the money quote:
Over time, however, other writers and scholars kept coming back to this incident, scrutinizing its consequences in ever more vivid detail.
The Durants spent an annoying amount of time on the Library of Alexandria. They made the observation that, just as Rome didn't fall for any one reason and didn't really fall at any one time, this "burning" of the Library at Alexandria is a retcon of the fall of Western civilization. There is no first-hand proof of the library's awesomeness and there is no evidence that anything destroyed there wasn't somewhere else; as mentioned in the article there are no less than four possible deaths of the library:
Some say, for instance, that Caesar’s fire merely destroyed a warehouse filled with scrolls and parchments recently unloaded from the harbor. The story of fourth-century Christian vandalism is attacked as a misinterpretation or a deliberate manipulation of the sources to serve an anti-religious agenda on the part of enlightenment scholars like Edward Gibbon. And Ibn Al-Qifti’s account is exposed as a myth written out of political expediency: as libraries across the 12th-century Muslim world were dispersed and auctioned off to pay debts, Al-Qifti created the legend of the bathhouse fires to stress that it was less of a crime to sell books than to burn them. In the end, no one was responsible for the library’s destruction. That always happened sometime else, at the hands of someone else. Perhaps, suggest the whispers, it had never really existed in the first place.
Ask Herodotus and the battle of Thermopylae was between 7,000 Spartans and a million persians; ask the Persiand and they'll say it was between 7,000 Spartans with a chokepoint and about 10,000 Persians with 90,000 mercenaries and wayfairers who weren't all that into it. "Alexandria" has been a touchstone for "back when we were geniuses" for more than 2,000 years.
We probably weren't.