The Talmud (Megilla 7a) uses this verse as a proof that Megillas Esther was written with Ruach haKodesh since Mordechai and Esther would have no other way to know that the holiday of Purim would never cease.
However, Midrash Shmuel explains that this verse is a prayer that Purim not be forgotten.
The Ginzei HaMelech writes that the holiday is forever because of Purim’s being the Jews’ re-acceptance of the Torah.
The Kedushas Levi writes that the holiday will never be nullified because Haman was from Amalek, and H-Shem promises “yad al keis ka” (“My Hand is on the throne of H-Shem”) (Shemos 17:16) that He will battle Amalek forever.
The Ohel Moshe notes that in the verse (Tehillim 137:5) “im eshkecha yerushalayim, tishkach yimini” (if I forget you, Yerushalayim, forget my right hand), Yerushalayim stands for Purim, and the right hand (yimini) represents Mordechai, who is called “ish yimini.”
The Baal Shem Tov writes that every generation will witness miracles.
On that note, R’ Avraham Yehoshua Heshel (Aptor Rav) and R’ Baruch of Mezibudzh explain the Talmud’s (Megilla 17a) warning against reading Megillas Esther “by heart” really means not seeing today’s miracles.
According to the Talmud (Yerushalmi Megilla 1:5) and the Midrash (Mishlei 9:2), even when other moadim (holidays) will go away in the time of Mashiach, we will still have Purim.
The HaSheol U’Mayshiv even explains the requirement to read Megillas Esther in the night of Purim represents Purim’s existence in exile, whereas the requirement to read Megillas Esther in the the daytime of Purim represents Purim continuing into the days of Mashiach.
According to Ohel Moshe, this also explains the difference between Jews and their seed mentioned in this verse. After all, are they not the same people? Rather, Jews keep Purim now, and their seed will do so in time of Mashiach.
On the other hand, R’ Chaim of Volozhin explains that Mashiach will come when all of the moadim (times) when he was predicted to come will pass. He will finally come when the same Jew-hate as existed in the time of Purim.
The Chafetz Chaim adds that the other holidays will not be literally nullified. Rather, we will give Purim more importance because it is the only time H-Shem saved the Jewish people from total destruction.
R’ Hutner explains that two people who are tasked with identifying a certain individual in the night. Giving one a flashlight would be a faster, more efficient method than training the other one’s ears to find the person. Although it is a good life skill, it is not the most effective method for accomplishing the task at hand. Similarly, the holidays provide light in exile in the relatively short-term. Purim, on the other hand, has the ability to train our senses to recognize H-Shem in nature, and that is an eternal possession.
The Dubno Maggid suggests that the reason why Purim will never cease is because the days themselves arouse the forces put into them during miracle. He provides an analogy of a king who is walking with two servants. If one were to become desperately thirsty, should the king send the remaining servant on the fastest horse in search of water, or should he order him to dig a well? From the perspective of the immediate, current situation, either option has equal potential. From the perspective of the future, however, whereas the water brought by horse has no future benefit, the dug well can provide water to other thirsty people for generations to come. A person desperate from thirst, upon finding the well, may even praise the king who ordered this well dug, for the act is enduring. By injecting certain periods of time, like Purim, with blessing from which we may benefit, H-Shem has inspired the greatest poet to sing (Tehillim 118:1) that H-Shem’s kindness “endures forever.”