Dealing with adversarial grant committee members in interviews
I am not in your field of work, however, as one of the comments to your question pointed out, this type of scenario can happen in other fields and situations.  The best first step is to acknowledge the debate and the limitations of your own study.  However, frame your response such that regardless of the debate, you will still learn something new.  For example, my response might be something like:

"Clearly there are two schools of thought as to whether quz or bar is the correct assumption, however, that question cannot be answered in a single study alone. By funding this proposal to do work under the assumption of bar, we can answer questions X, Y, and Z.  An additional outcome is that we will be providing more information about phenomena surrounding bar, thus providing more information that may ultimately help to answer the question of quz versus bar."