Even if Esperanto were widely adopted, there would be nothing to keep it from developing edge cases and funky grammar exceptions over time just like every other language.
Esperanto only works for scholarly types that adhere strictly to the rules, not lazy uneducated people who just want to communicate something with as little effort as possible and don't care if they aren't adhering to the grammar. Also, you'd get localized variants depending on the region.
The process of languages acquiring idiosyncrasies takes a really long time. I'd suspect it's even slower for auxiliary languages. Esperanto would likely remain relatively simple for centuries. Just compare English from the time of the British Empire to now. It's still basically the same language, despite (or because of?) the huge number of people learning it as a second language.
The lazy uneducated poets would butcher the language first, followed closely by the lazy uneducated lawyers. The day might come when lazy uneducated CEO's pervert it even further, but I'm gonna have to circle back with you on that and touch base later.
Esperanto only works for scholarly types that adhere strictly to the rules, not lazy uneducated people who just want to communicate something with as little effort as possible and don't care if they aren't adhering to the grammar. Also, you'd get localized variants depending on the region.