I think I would take you up on that last point. Definitely there is a lot that survived that is great (and some trash), but a lot of the stuff that got forgotten is also potentially good and a victim of circumstance. Music scores were not as widespread as books and so wars (especially modern ones) took their toll on many libraries in Europe.
Also, partly it was due to the 'in the moment' nature of music. You can read a book several decades later, where as to hear a piece of music years later there is much more effort needed by a great number of people to have that chance, and if anything, the concert organisers (both historical and contemporary) were notoriously conservative in their programming. Also, the idea of listening to music from the past was only a recent phenomenon that only began from Mendelssohn.
You're right - some excellent material gets forgotten. Vivaldi was almost forgotten. I've always loved history, and I find that old music by artists like Palestrina can make you feel transported back in time. Although that fantastic music by Valente, whom I'd never heard of, sounded quite modern!
I find that classic novels can transport me to an earlier time and place too.