Amazon would not be able to monopolize the sale of ebooks if it was legal to distribute them for free, people would just go download the free versions elsewhere. This is already more or less reality; you can easily go download most any book for free, and have effectively no risk of getting in trouble for it, yet somehow most people do not do this.

I think that if an author provides a way that people can pay them directly for having written a book, social convention could take care of the rest, many people will give them money voluntarily, just like they currently do, because they want to. There's lots of platforms now that operate on the principle that people will just choose to give money to creators even when they don't have to and it isn't a purchase of anything tangible.