If one person is reasonable and the other person isn't reasonable, then the reasonable person loses.

This is usually true, yes. If one is not dependent on the unreasonable person for the outcome, or it is worth the conflict as the desired goal is necessary, it often nullifies the unreasonable persons insertion. Often, one can use the others lack of control to nullify them to others who matter, or better, educate those on the fence when they see for themselves an unreasonable opposition. Bypassing such people is the best option if possible.