In the Rambam Edition linked by Double AA the bene Binyamin commentary reported a Talmud Yerushalmi in Megilah Chapter 1 halacha 5.
ר' יוחנן ורשב"ל ר' יוחנן אמר הנביאים והכתובים עתידין ליבטל וחמשה סיפרי תורה אינן עתידין ליבטל מה טעמא (דברים ה) קול גדול ולא יסף רשב"ל אמר אף מגילת אסתר והלכות אינן עתידין ליבטל נאמר כאן קול גדול ולא יסף ונאמר להלן (אסתר ט) וזכרם לא יסוף מזרעם הלכות (חבקוק ג) הליכות עולם
The Yerushalmi exposes two views, Rabbi Yochanan thinks that Pentateuch only will remain and Resh Lakish adds Megilath Esther.
The words pf Rambam are based on this text, following Resh Lakish's opinion.
The Or Sameach refers to a Midrash on Kohelet which says that the books of the King Salomon are not relevant knowledge but advisories against sin.
Most cpmmentaries refered to a Gemara in Nedarim 22b which says similar things about Prophets an agiographs excepted sefer Yehoshua ( which describes the partition of Erets Israel).
Lechem Mishne explained that the Raavad who reject the Rambam's statement mainly rejected thelack of delimitation of holly books cancellation.
Anyway, this matter is not a consequence of the a personal opinion of Rambam regarding Esther book. As almost all halachot in this book this halacha is a Rambam's paraphrase of Chazal conclusions. May be that most people know this but I want to make it clear.