I think this is a somewhat wrong framing, and its also shitty to blame libraries for this shift. Tech companies, for the most part, are responsible for the destruction of attention spans, if that has really happened. And I'd be happy to bet that by whatever criteria you choose to select there are more great books written per year now than in 1240 or whatever time you think they only wrote great shit. Its just that now there is much more to wade through and the media environment is totally different.

At any rate, I just think that its a very strange thing to do to use "old" as a substitute for "good." There are tons of old books that are moronic and if the population of the world back then had been the same as now there would be tons more.