Is there a legal alternative to illegal projects like Libgen? I would really really want something like Netflix for books, where I can easily discover and read books.
Libgen and the like tend to just have more on hand though, and that's the big differentiator in usability IMO. There are things your local library just isn't going to have a copy of but libgen will. After that happens once, why bother with the library again? Outside of "it's legal" or "I find more moral" type concepts there tends not to be a strong reason.
Plenty of books (and other written works, such as serial publications) are in the public domain, hence fully legal from a copyright POV. However discovery is still a major problem: many works in the public domain are still far from being easily findable or accessible online. (Even then, it's worth keeping in mind that the books people generally think of as the 'Greatest Books of the Western Canon' are, by and large, in the public domain, and that already is more books than you could feasibly read in a lifetime.)
also kindle unlimited
Libgen is not Netflix for books, it is thepiratebay for books. Libgen is not helpful in discovering more books because if to judge about those literature which is abundant on Libgen, the technical one, what allows user to discover some books on Libgen is only another books or your interest to specific scientist or field.
(I know there are a lot of fiction materials on Libgen such as comicses but all I use to read is science books or at least some non-fiction, so my opinion may be biased).