As far as RWD taking twice as long to design and implement, perhaps this is just a question of experience. Once you've had some practice with RWD, although it does take some extra time, I don't think double the time is typical.
The argument that only a few of their customers are on mobile is the strongest reason presented not to use RWD, but the question I would be asking is, "Is this situation likely to remain that way? Will mobile users only ever be 2% of our traffic, or can we expect that this proportion will climb?"
Personally, I would expect that this proportion would in fact increase over time, but I don't have a crystal ball and can't say this for certain.