By capturing Russian territory Russia now has a reason to come to negotiations to just call everything off to get their land back.

Ukraine captured land in kursk and this did not cause the Russians to come to the negotiating table, nor signal that they will weaken their demands. In fact, they simply started taking land even faster in Ukraine because Ukraine had committed resources into kursk instead of the front lines.

All it goes to show is that westerners have a complete non-understanding of this war.