And in this way all Israel will be saved, as it is written, “The Deliverer will come from Zion, he will banish ungodliness from Jacob”.
Romans 11:26 ESV

The hardening of Israel wasn’t just to encourage the grafting in of Gentiles, but to hasten the redemption of Israel. If they were allowed to go on as they were, they would have grown in their pride and indifference.

The hardening of Israel leads to exile, which leads to suffering, which leads to repentance, which leads to restoration. The silversmith doesn’t remove dross from the silver by being nice and affirming. If he cares about the metal at all, he refines it through fire.