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.
I find responsive design a cheap copout, jack of all screen resolutions, master of none.
Just create a good desktop design, and make sure it plays on a modern smartphone with tap-to-zoom. Tablets will work automatically.
I dont'see "more diversity of screen sizes" coming affecting this at all. New screen sizes will just be bigger than an 2010 iPhone/Android phone. Those can play well with desktop-sized designs anyway.
And even if you have a 30" monitor you still don't need more than 1000-1200 pixels of webpage estate anyway. For one, people can't read or follow freakishly long lines of text easily.
I'd be interested to learn more about their design and implementation workflow, that they mention RWD taking 'almost twice as long'. How much time are they wasting building static comps that illustrate a picture of what some people will see when they come to their website? This is speculation - I don't know how much live prototyping they do in their design process.
I'll have to mention Edit Room again, my web design tool that makes creating responsive, multi-screen web layouts as easy as dragging things around on-screen. If you start your design work in the browser, you save so much time that lets you go that extra 10% on a design. That gives you the time you need to make your designs work everywhere.
http://www.edit-room.com