More and more out of print old books are being put online every day. I’ve found so many that can be downloaded for free. I’d have never found them otherwise and certainly never have been able to afford them.
You have observed random and limited reproduction of such material, not blanket availability. The point I am making is that books can be read as long as the paper survives. Electronic media’s lifetime is at the mercy of the marketing people and the transient nature of the media itself. To keep anything a long time one must transfer content from media to media, forever and often.