I think it’s more the “assume a perfect spherical cow” style of argument. It’s true that people will stop buying if the shipping price hits some incredibly high level but it’s a complex system where driver pay is just a small part. The most likely outcomes are things like raising their own prices, becoming more economical in their use or packaging, shifting the delivery times & intervals, etc. — things which happen all of the time without reaching such dramatic levels. The data I’ve seen has the cost of fuel being right behind compensation in cost and that fluctuates all the time without people halting purchases.