This Plan B is YOUR delusion.
YOU are the one asserting some sort of a Plan B - that God turned to the Gentiles when Israel failed this supposed mission of yours.
Fact is, the mission in the OT is to bring worship from all over the world to and at Jerusalem.
Isaiah 2:1 The word that Isaiah the son of Amoz saw concerning Judah and Jerusalem. 2:2 And it shall come to pass in the last days, that the mountain of the LORD's house shall be established in the top of the mountains, and shall be exalted above the hills; and all nations shall flow unto it. 2:3 And many people shall go and say, Come ye, and let us go up to the mountain of the LORD, to the house of the God of Jacob; and he will teach us of his ways, and we will walk in his paths: for out of Zion shall go forth the law, and the word of the LORD from Jerusalem. 2:4 And he shall judge among the nations, and shall rebuke many people: and they shall beat their swords into plowshares, and their spears into pruninghooks: nation shall not lift up sword against nation, neither shall they learn war any more. 2:5 O house of Jacob, come ye, and let us walk in the light of the LORD.
It is not Paul's mission - which is NOT a Plan B.
Rather, Paul's is a different mission altogether.
Paul's concerns that other aspect of God's Two-Fold Purpose: the Heavenly.
Two aspects of ONE "plan" - "IN Earth AS IN Heaven."
Ephesians 1:8 Wherein he hath abounded toward us in all wisdom and prudence; 1:9 Having made known unto us the mystery of his will, according to his good pleasure which he hath purposed in himself: 1:10 That in the dispensation of the fulness of times he might gather together in one all things in Christ, both which are in heaven, and which are on earth; even in him: 1:11 In whom also we have obtained an inheritance, being predestinated according to the purpose of him who worketh all things after the counsel of his own will: 1:12 That we should be to the praise of his glory, who first trusted in Christ.
Ephesians 2:6 And hath raised us up together, and made us sit together in heavenly places in Christ Jesus: 2:7 That in the ages to come he might shew the exceeding riches of his grace in his kindness toward us through Christ Jesus.
ONE Two-FOLD Purpose IN Christ...
Ephesians 3:15 Of whom the whole family in heaven and earth is named,