The Sages teach that Moses thought his sons would take his place leading the people, but God informed him that Joshua would be his replacement.

Moses believed that his sons would inherit his place and assume his position of leadership, so he requested from the Holy One, blessed be He: “May the Lord appoint a man over the congregation” (Numbers 27:16) who has leadership qualities. The Holy One, Blessed be He, said: Moses, it is not as you believe. Your sons will not inherit your position. You know that Joshua served you and accorded you great respect, and he would rise early and remain late in your study hall to arrange the benches and lay out the mats. He will assume the leadership, to fulfill what is stated: “The guardian of a fig tree will eat its fruit” (Proverbs 27:18). (Yalkut Shimoni, Pinhas 1:777)