L.L. Zamenhof's original dream was that other languages would eventually die out and be replaced by Esperanto or something like it. He came from a time, place, and personal perspective when "diversity" wasn't valued like it is today. (Similarly, he dabbled in creating a universal religion that adherents of other religions could compromise towards.) It wasn't until after the mid-20th century that the Esperanto movement started talking about supporting the world's minority languages, and even then not all parts of the organized movement got on board.
From a sociolinguistic view, it seems likely that if Esperanto were the universal language, minority-language speakers would abandon their native languages for it in many circumstances ("domain loss") just as they do for English, French, Spanish, etc. in various parts of the world.
L.L. Zamenhof's original dream was that other languages would eventually die out and be replaced by Esperanto
I think you're doing him a bit of a disservice here. His idea wasn't to kill other languages at all. He felt that if you only had to learn Esperanto to communicate with the rest of the world, you would, in fact, be able to spend more time enriching your own language. As he wrote in "Unua Libro":
"How different would the case be, had we but two languages to learn; we should know them infinitely better, and the languages themselves would grow richer, and reach a higher degrees of perfection than is found in any of those now existing"
The Unua Libro was written (naturally, as its title shows) early in Zamenhof’s activity. In later years, he went further and further down the path of "all humanity being one" rhetoric.
Interesting. This seems counter-intuitive for someone who spoke many languages and apparently delighted in their diversity. I also agree with the comment above yours that wanting humanity to "be one" and to be able to communicate universally, is not mutually exclusive with them maintaining their separate languages concurrently. Do you have any suggested sources for me to read about Zamenhof's later views?
My understanding of Zamenhof’s views is based on reading his complete works in the edition prepared by that Japanese Esperantist, which I could do as I was closely involved with an Esperanto library. As this was some years ago and I am no longer involved in Esperanto, I am sorry that I cannot cite a specific passage, but I am quite sure that my claim is correct, and I would urge you to go to those sources.
From a sociolinguistic view, it seems likely that if Esperanto were the universal language, minority-language speakers would abandon their native languages for it in many circumstances ("domain loss") just as they do for English, French, Spanish, etc. in various parts of the world.