Why would the full inclusion of the Jews be a blessing to the Gentiles?

In Romans 11:12 Paul again returns to the desire of his heart, namely, that his family and kinsmen will also share in Christ (see also Romans 9:1–3, Romans 10:1). He hopes that the work he has done among the Gentiles and which has been blessed by God, will ultimately also bring about Israel’s salvation. Here he again describes just how passionately he desires this: Now if their trespass means riches for the world, and if their failure means riches for the Gentiles, how much more will their full inclusion mean! In the Greek, Paul does not specifically mention their conversion. The word translated as full inclusion follows two negative words. The first is trespass, which is connected to what is written in Romans 11:12. The offence against the Son is Israel stumbling. This is then followed by the second negative word, namely failure, which is how the ESV translates the Greek word hettema which literally refers to being diminished and defeated. Israel is diminished because of its transgression and unbelief, thereby losing its prominence even while the Gentiles are brought forward. Over against both tresspass and failure Paul now places full inclusion. Cranfield¹ denies this relationship, arguing that hettema only means defeat and has nothing to do with being diminished. Yet even when the meaning of defeat is accepted, the clear contrast with pleroma (which literally means fullness) still remains, for the full blossoming of Israel presupposes that the transgression has been paid for and that the people have survived the defeat (or diminishing). Israel will reach this fulness when it no longer desires to suppress the Messiah but allows the fullness of his light to shine among themselves as a people. Only then will Israel once again be, and even more that before, a guide to the blind, a light to those who are in darkness, an instructor of the foolish (see Romans 2:19–20). If the solar eclipse passes by, the nation from which the Messiah was born will itself serve as the sunlight of the world—the full light for the nations.²