Many good answers. It would have made a difference, maybe of a few hundred years, but we probably had regressed before that and probably will again. Many questions would have been answered, but could we really answer the BIG ones? Was there a record of our development as humans, both culturally and technologically, stretching much further back in time? Was there a "mother culture" that had been destroyed as in so many myths but left some tangible record, maybe yet to be deciphered, in Alexandria's library? Could we be migrants from somewhere else?
The loss of Alexandria was due to a combination of conquest and ignorance. Knowledge was preserved through the Dark Ages by being confined to preistly and shamanic classes. Our knowledge is today disseminated to the point where so much is known by so many. However, so little, as they say, is cast in stone, and exists only in a virtual environment. Natural cataclysms can wreak as much havoc today as they did in ancient times, and were a destructive event to visit us on a cosmic scale, our knowledge might again be reduced to where it was after Alexandria was sacked. Pyramids would outlive our grandest cities and clay tablets, our greatest works of literature and art. And someday, hundreds of years from now, someone else will be having this same discussion.