Their point is that when they try and trade in their 3 year old truck which they got a low interest rate for a new one, not only is their truck worth bugger-all

Their point is that when they try and trade in their 3 year old truck which they got a low interest rate for a new one, not only is their truck worth bugger-all to trade in but their next interest rate will probably be doubled or more. Hence they'll stick with their current one so one less sale.