I agree that the estimate of amount of time it takes to make their site responsive seems high to me. Even if it's not perfectly designed it's better than not being responsive at all, and getting it pixel perfect will probably be cheaper and reasonably performant as designing a mobile-only experience.

However I disagree that making a design responsive today is an investment for the future. Designs change much quicker than demographics, so if they don't have significant mobile traffic today it's unlikely they'll have it during the lifetime of the design.