Paul interprets Israel’s “stumbling” as not completely permanent. Israel’s present stumbling over the stumbling-stone (Christ) benefits the Gentiles. Thus: “through their stumbling salvation has now come to the Gentiles” (Rom. 11:11).
In essence, Paul’s argument is this: Israel’s stumbling leads to Gentile inclusion in the Messiah’s family, and this Gentile inclusion provokes “Israel” to jealousy, and eventually (see Rom 11:26) to faith and salvation.
But what Paul hints at in Rom 11:12 is that Israel’s present diminishment will not only lead to Gentile “fullness” but also — to an even greater degree — to Israel’s eventual “fullness.”