John Gill Commentary Romans 11:19

"Thou wilt say then, Branches were broken off, that I might be grafted in." — Romans 11:19 (ASV)

You will say then
This is an objection which the apostle foresaw the Gentiles would make against what he had said, and in favour of their boasting;

the branches were broken off, that I might be grafted in .
The sense of which is, that the Jews were rejected and left out of the Gospel church, on purpose to make way for the Gentiles, that they might be put in their room; and consequently the Jews must be more vile and unworthy, and the Gentiles more deserving of such favours and privileges, or God would never have taken such a step, to leave out one to make room for the other.