If Esperanto ever became the dominant international language people would start making blockbuster movies in it, kids would start learning it, and younger people in smaller countries would use it more than their native tongue.
Then it would start to displace a few native languages until it became some community's native language.
After that it would start to fragment and evolve and lose all the simplicity that comes from it being an artificial language.
I don't think it would fragment that quickly. In times past travel was slow, there was no telephone, or news papers. The few books were mostly reserved for the elite.
600 years ago (before the printing press) most people lived either on or near their farm. You went to the nearby village for things you couldn't make on the farm. Traveling to the next village was as far as most people could go: they needed to get back to the farm to milk the cow again, or otherwise care for the farm. As such there was no way to know your language was fragmenting, much less any reason to care.
Today we have printing presses, telephone, TVs, movies. All give us ways to find out about fragmentation and reasons to care. While languages will still change and fragment over time, the above pressures will help to keep the changes in check.
Language in a literate society changes more slowly. But it still changes, given enough time it will fragment.
There's also no reason to think Esperanto would evolve or fragment any slower than English once it's actually in use. Which negates many of the arguments that we need Esperanto as a global lingua franca because English will fragment once America declines.
English speakers traditionally hate the idea of language planning, at least if it's an overt governmental program, but it's had some powerful effects elsewhere.
and some kind of mechanism to enhance the language regulator's influence (although I don't know exactly what that would be).
I don't think that Zamenhof thought this aspect through in much detail, because he thought that people would continue to use Esperanto exactly as described by the Fundamento de Esperanto. See paragraph 4 of the Boulougne Declaration:
> [...] The only single, perpetually obligatory foundation of the language Esperanto for all Esperantists is the work, Fundamento de Esperanto, to which no one has the right to make changes. If someone deviates from the rules and models from the above-mentioned work, he or she cannot ever excuse himself/herself with the words: "so desires or advised the author of Esperanto". Every idea that cannot be conveniently expressed by the contents of the Fundamento de Esperanto, all Esperantists can express in a manner which they deem the most correct, as is done in any other language. But for reasons of unity all Esperantists are recommended to imitate, as much as possible, that style which is found in the works of the creator of Esperanto, who has worked more that any other for and in Esperanto and who knows its spirit better that any other.
However, I don't think this has worked out exactly as Zamenhof intended, because I think there have indeed been unofficial changes and divergences, as well as slang (maybe smaller than those you'd expect to see in a non-constructed language, but still some). Also, I think there are ultimately grammatical and usage questions that the Fundamento de Esperanto didn't address, because Zamenhof wasn't quite a linguist in the modern sense, didn't know about certain grammatical issues, and didn't conduct user testing with speakers from different linguistic backgrounds.
Then it would start to displace a few native languages until it became some community's native language.
After that it would start to fragment and evolve and lose all the simplicity that comes from it being an artificial language.