IMO, you are both right!
You are right, in that ANY use shortens the service life of any machine, just as humans begin dying the minute they are born. Of course, the RATE of wear varies according to use. Will the life expectancy be noticeably shorter if the rig is often used at or near (or even slightly above) the rated capabilities? I believe most people won't own the vehicle long enough to notice. the third or fourth owner might wonder, though.
Your DW is right in that it is a truck, designed and built to do work. If it is properly maintained, and used within the manufacturer's guidelines, it should last quite well.
Good luck.
