And often mobile is an extension of the core product not the product itself. While this is true and you do have a point, the reason for this may is not always

And often mobile is an extension of the core product not the product itself. While this is true and you do have a point, the reason for this may is not always obvious. Sometimes it makes sense for mobile to be an extension of the core product. Other times mobile is an extension of the core product because redesigning for mobile turned out to be a bigger task than anticipated because the product was not designed for mobile in the first place. Not saying I agree that you should always do mobile first, but just pointing out that both of you may be right at the same time.