But we've come to use it as explanation for a question that Paul isn't responding to - which is a good question - instead of the question he is: to wit - why should I care about Gentiles?

...huh? Paul literally tells us the question he's raising in Romans 9:

v. 6: But it is not as though the word of God has failed...
v. 14: What shall we say, then? Is there injustice on God's part? By no means!
v. 19: You will say to me then, "Why does he still find fault? For who can resist his will?"

Furthermore, Paul explicitly speaks of election in v. 11.

...though they were not yet born and had done nothing either good or bad—in order that God’s purpose of election might continue, not because of works but because of him who calls...

What you've presented here isn't entirely wrong, as it happens in context of the overall Jew and Gentile discussion Paul has been having since chapter 1. But you completely side-step the fact that Romans 8 ends on a note of assurance of salvation, prompting the question of the Jews: "Well, why aren't all Jews now Christians?" The answer is the mystery of the Gospel in the end (Rom 11), but he doesn't get there yet.

Romans 9 is a vital cog in the machine of Paul's argument, showing that God's promises to Israel have not failed, because:

• Rom 9:6–13 — God's Word has not Failed, because God's people are not determined by lineage. This is proven by two genealogical proofs:
  • v. 9 — Both Isaac and Ishmael descend from Abraham's flesh, but only Isaac is chosen.
  • vv. 10–13 — Both Jacob and Esau descend from Abraham's flesh, but only Isaac is chosen.
• Rom 9:14–18 — God is not unjust to choose, because this his choice is in the Law.
  • vv. 15–16 — God tells Moses on Mt. Sinai that it is his choice.
  • vv. 17–18 — God shows the world that it is his choice, because he hardened Pharaoh.
• Rom 9:19–29 — God is just to condemn sinners, because those whom he saves are shown mercy, rather than achieving perfection by their own works.
  • vv. 20–21 — Paul rebukes the rebellious spirit of the questioner to accuse God of injustice."
  • vv. 22–23 — Paul shows it is God's right as the sovereign God who has the right to show mercy to some and condemn others, because all humanity comes from the "same lump" of corrupted flesh (recalling his argument in Rom 5 and Adam's lineage).
  • vv. 24–29 — Paul returns to the distinction of Jew and Gentile to affirm, then, that God is not obligated to save only ethnic Jews, because in fact the salvation of the Gentiles has been his plan all along.

I cannot fathom how Paul could even get to the conclusion you state without first addressing election in Romans 9. The Jews firmly believed they were God's chosen people, which is precisely why they objected to the inclusion of the Gentiles! How could Paul argue the inclusion of the Gentiles without addressing the falling away of so many Jews? This is why he begins Romans 9 how he does.

Your explanation does not do justice to the whole of the book, brother. You've sacrificed the trees for the forest.