The reason that many poskim ruled against this type of prenup, is because the text of the prenup purports to compel the husband to give a Get in cases where he is not halachicly obligated to do so. And it enforces this with a daily monetary penalty, another halachic problem where he is being forced against his wishes to provide the Get.
They therefore rule that a Get provided under those circumstances results in an invalid Get, and any remarriage by his wife is adultery and future children mamzeirim.