I was told by a Jewish colleague of mine that the Mosaic covenant was not promised perpetuity?

The Mosaic covenant was conditional. If the Israelites didn't fulfill their duty, then God would punish them and withdraw his presence from the land (see the curses in Deuteronomy 28). Notice that the Abrahamic covenant, in contrast, was not dependent on Israel's faithfulness (Genesis 15). This is why the Mosaic covenant had to be renewed several times in the Old Testament (after exile, for example), but the Abrahamic covenant did not. As a Christian, I believe the Abrahamic covenant was fulfilled in Christ himself, so now the Mosaic covenant is irrelevant.