והיה כי תמצאן אתו רעות רבות וצרות וענתה השירה הזאת לפניו לעד; "it will be when many evils and distresses come upon it, then this song shall speak up before it as a witness.⁠" This is a reference to the song at the beginning of Parshat Haazinu. There the Torah speaks in detail about the ones who have been cast out.

כי לא תשכח מפי זרעו כי ידעתי את יצרו אשר הוא עושה היום, "for it will not be forgotten from the mouth of its offspring; for I know its inclination, what it does today.⁠" Moses says that he can draw conclusions from Israel's behavior in his own time when the people suffer from an inclination to practice idolatry. It is likely therefore that they will put their inclination into practice in the future. This is why, when the penalties described here, i.e. the sufferings, will occur, they will find that they have all been foretold in the Torah. The reason they will find this out is that the Torah will not be forgotten among their offspring (despite their committing all these sins). The witnesses in the song in the Torah in which heaven and earth are called on will endure and give their testimony.
The reason Moses was allowed to say such words as "for I know its inclination,⁠" [in a negative connotation, and God did not punish him for slandering the people as He had done in Exodus when he had said they would not believe him (Exodus 4,1), Ed.] is that God Himself had described the people in these terms in Psalms 103,14 where the psalmist says "for He knows our inclinations.⁠" Midrash Tehillim on this verse states; "woe to the dough of which the baker himself says that it is defective.⁠"

It seems to me that the wording of the Torah does suggest that there would come a time when the Torah would indeed be forgotten by the people. The time in question is when, in the future, the evil urge will be annulled, as distinct from the messianic era during which, as we pointed out previously, (Shabbat 138) "business will be as usual,⁠" all the commandments of the Torah remaining applicable. As long as this present universe continues, Torah will continue as the guideline for the Jewish people to live by. The period when Torah will be "forgotten" will be during the period of the resurrection. In other words, as long as the people have the same inclination as now, i.e. they are burdened with the evil urge, Torah will not be forgotten by them. Once they are free from the evil urge there is no need to remember Torah. At that time the only holiday which will still be celebrated will be that of Purim, (seeing that it came about as a reward for the Israelites who had done תשובה).

The foregoing is based on a Midrash Mishlei 9, where the sages comment on the verse in Proverbs 9,2: טבחה טבחה, מסכה יינה, אף ערכה שלחנה, "she has slaughtered the meat, mixed her wine, she has also set her table.⁠" The sages understand these words as describing the time when Queen Esther prepared the meal for the King and Haman. The words: "she also prepared her table,⁠" which appear superfluous, mean that she prepared for the life in the hereafter as well as for that on earth when she took those fateful steps. The emphasis on this meal, i.e. a festive meal, is an allusion to the time in the future when all the festivals and their festive meals will be abolished, although the festive meal on Purim will not be abolished. This is the meaning of the words at the end of Megillat Esther 9,28: "and these days of Purim will not disappear and their memory (what occasioned them) will not be forgotten by the Jews or their offspring.⁠"