Doesn’t that mean that the Church is the true heir of this covenant?
Like the Abrahamic covenant and other covenants God made with Israel, the New Covenant was not conditional upon Israel’s obedience or acceptance of their Messiah. It was an unconditional promise given to Israel because God is gracious. As Romans 15:8 indicates, God will keep His promises that He made to the patriarchs of Israel because He is pleased to do so and He is a God of His Word. The Church is made a joint heir with Israel of at least the Abrahamic and the New Covenants, but does not replace Israel and take over her destiny. The Church is a surprise addition to the prophesied program of salvation promised to Israel, and is both included in the blessings to Israel and given blessings of her own. In regard to Israel and the Church, God treats the situation as “both / and” rather than “either / or,” at least in the long-term sense.