There are two problems with understanding this Rashi. One is that the Gemara tells us that the reward for mitzvahs is not in this world. While it is true that HASHEM rewards every good a person does, the place of that reward is in the World to Come. In fact, it is considered a curse to use up your payment in this world – something that is reserved for wicked people. So it doesn’t seem to be correct that their livelihood was dependent upon listening to HASHEM.
The second problem with this Rashi is that any motivational system must be tailored to fit the audience. The people of this generation received the Torah on Har Sinai. They spent almost forty years surrounded by the Clouds of Glory, completely immersed in Torah study, and sustained by the mon. They were on the highest madreigah of any generation in history. So even if their parnassa was dependent upon their listening, how would they be motivated by something so mundane as earning their daily bread?
