It is because of Hashem that B'nei Yisrael are, and will be, sustained. Because of their abandoning Hashem they will be punished, invaded and nearly destroyed. Moshe does promise that, inevitably, the Ribono Shel Olam will redeem His chosen people and be close to them once again. But he stresses that the Jewish People will eventually forsake Hashem. His final command, which requires the teaching of the Torah, seems to only be a preventative measure.
Moshe leaves no hope.
The Jews may stave off their fate for a time, but his prophecy seems to suggest that the exile is unavoidable. The answer seems to come from Moshe's very introduction to this prophecy. The deeds of the Mighty One are perfect, for all His ways are just. Hashem promised Avraham Avinu the land of Israel just as he promised it to Yitzchak, Ya'akov and Moshe. But the promise was not unconditional.