Let Reuben live, and not die. — “‘Live’ in this world.” says Rashi, “and ‘not die’ in the world to come.” That his misdeed should not be remembered (Genesis 35:22). Rashi also notices the juxtaposition of this record with the sentence, “the sons of Jacob were twelve.” Reuben was not cut off, but he was disinherited (1 Chronicles 5:1), and his father’s blessing had so much in it of disapproval, that Moses’ prayer for him was not unnecessary.
And let not his men be few. — The sentence is difficult. The LXX. insert Simeon, “let Simeon be many in number.” But there is no need for this. The most terrible destruction ever wrought in Israel by the word of Moses came on Dathan and Abiram (who were Reubenites), when “they and all that appertained to them went down alive into the pit.” We cannot say how far the tribe was diminished by this terrible visitation and the plague that followed (Numbers 16), but the fighting men of the tribe had slightly decreased in the second census (Numbers 1:21; Numbers 26:7), and only two of all the twelve tribes had a smaller force than Reuben
at this time. It seems best, therefore, to take the whole verse as applying to Reuben, and the negative in the first clause as covering the second clause also. “Let not his men be a (small) number.” The omission of Simeon may be accounted for by his coming within the inheritance of Judah, in Canaan, and enjoying the blessing and protection of that most distinguished tribe. Rashi also takes this view.