Listening to yesterday's first reading, I noticed how the LORD's kvetching about His people serves as a test of Moses' faith.
As a descendant of Israel, Moses could be made the patriarch of a great nation, as God offers to do in Ex 32:10, and technically God would still be keeping His promise to Abraham.
But Moses passes the test. Contrary to the rest of the Israelites, who are at the moment worshipping a golden calf, he has a true understanding of the LORD, Who does not make promises lightly nor interpret them narrowly. If the LORD made of Moses a great nation, then Abraham's faith would become a historical curiosity, like Enoch's. It would be from Moses, not Abraham, that God would build His people, and though no one would have standing to call Him on it, to do so would be for God to go against Himself.
Moses' faith in his God will not allow him to accept this, and the LORD, finding one man of faith among His people, spares them all.