In this week’s parshah the Torah teaches the halachos of an ir hanidachas – a city where a majority of the inhabitants serve avodah zarah. The halacha is that all of the city’s inhabitants are killed and all of their possessions are burned.
On Purim we read Megillas Esther twice, once by night and once by day. It is uncertain what the nature of the obligation is.
Why is Purim eternal? I would suggest that Purim is the prototype of the End of Exile, and as such will remain forever linked to the Era of Mashiach that will occur after the Final Redemption. Therefore, Purim will be part of that entire period at the culmination of history.