Cantoresq,Megillas Esther is part of the Mesorah. The books of the Maccabees are NOT. They were written to play up the Hasmonean family. Taanis Esther may be a relatively new observance, but Purim isn’t. Both from a Torah point of view and an irreligious academic point of view, the holiday of Purim is well-established.
The holiday of Nicanor in no way, shape or form contradicts a holiday established much earlier. Heck, even the additions to megilas esther are dated earlier than the hasmonean revolt. Josephus Flavius, the ancient equivalent of a Jew calling himself “Joseph Hitler”, had to admit to Purim’s existence.
Finally, the book of Maccabees upon which your question is based actually says that Nicanor day is the day before Mordechai’s day, aka Purim:
35
And he hung up Nicanor’s head in the top of the castle, that it might be an evident and manifest sign of the help of God.
36
And they all ordained by a common decree, by no means to let this day pass without solemnity:
37
But to celebrate the thirteenth day of the month of Adar, called, in the Syrian language, the day before Mardochias’ day.
38
So these things being done with relation to Nicanor, and from that time the city being possessed by the Hebrews, I also will here make an end of my narration.
Ad kan. This thread is drifting dangerously close to haskalah.