It was a speculative statement. My reading of Romans 11:25-27 is that Paul hoped that his people would repent en masse, as Israel, after the disaster of the war against Rome—that is, after judgment. If that had happened, things may have worked out very differently.
Would restored Israel, under a new covenant, with Jesus as Lord and Messiah, have continued to observe the Law? Paul’s argument in Galatians is not against Jewish Christians observing the Law but against them imposing the Law on Gentiles.
I don’t think it’s inconceivable that the people of God would have retained a distinctly Jewish identity, with the markers of the Law in place—sabbath observance, circumcision, festivals, etc. None of that was incompatible with the indwelling Spirit presumably if the early believers continued to live as Jews.
It would still have been necessary to find a way to accommodate Gentiles, but would the church have become a Gentile movement if all Israel had been saved? It’s arguable that Paul sees the inclusion of Gentiles as a means of making Israel jealous rather than as an indication that the future of the people of God was to be overwhelmingly non-Jewish.
We might then have a situation rather closer to the Old Testament vision of the nations not so much being included in Israel as coming to learn the ways of Israel’s God, much of which would be found in the Law and its practices. The whole Christendom fiasco might then have been avoided!
But as I say, it’s just historical speculation.