First off, The pre-nup has the endorcment of Rabbis Zalman Nechemia Goldberg, Dov Schwartz and Moshe Heinemann, who I do not believe are MO.
Second of all, why does it matter that most of the rabbonim are MO? Rabbis Willig and Schachter are both gedolim. Why should their opinions matter less?
Finally, the pre-nup is widely used. The RCA released a statement saying that none of their members should officiate a wedding if a pre-nup was not signed. All the YU roshei yeshiva support the pre-nup.