Additionally those are very good points about English and streaming. There are books out there which are not translated and not digitized, which are absolutely worth reading, and will not be found anywhere besides private collections, used bookstores, flea markets, estate sales. Some of these books were originally purchased long ago and/or far away, just because they are not popular enough to be easily found does not mean they are not worh finding.
I'd be curious how this compares with LibGen / ZLibrary (about 4--5 million titles).
https://news.ycombinator.com/item?id=31604033
Keep in mind that a largish municipal library is on the order of 250-500k items.
The Library of Congress includes ~40 million catalogued books, of which ~360k circulate annually.
Again, the question is what Amazon offers.
Then you should broaden your imagination.
Amazon and Goodreads have barely a fraction of one percent of the books that were published. And that's just in English.
It's like saying "Anything worth watching is on streaming," even though less than 5% of the world's video content ever made the translation from film to VHS to DVD to streaming. Or paper to records to tape to CD to streaming.
There's a whole vast world of content out there beyond the internet. Once you discover this, it's like taking a pill in The Matrix.
(My metaphor might be off there, as I've never seen The Matrix, but from what I understand from other people's conversations, that should work.)