I just want to encourage the practice of authors allowing everybody to read their books. Uploading it to libgen kinda defeats the purpose.
But I don't really agree with you. The author will do what the author wants to do, it has no bearing on somebody else uploading something he bought to libgen or not. I view libgen as a tremendously valuable resource and I try to contribute as much as I can to it. Not everybody who could benefit from a book can afford to drop 10 or 20 or even 5 dollars on it.
I actually proposed the administration of my lab to stop paying all silly online journal subscriptions and give half of the money to libgen and the other half to scihub. Everybody seemed to mostly agree, but in the end there are "high level" influences that block this kind of things.
I also think that it is perfectly moral and ethical to share a file that you have on your computer with other people, regardless of the opinion of the author of the file.
My only concern is that, if my main goal is to convince an author (who is already sharing a large part of his work) to publish the whole thing, it may not be the best idea to start by uploading his file to libgen. The fact that it is on libgen will not make me buy it in hard-copy. But if he puts the .tex sources online, then I will buy it.