I don't think the mileage is necessarily a good reflection of the wax's durability. Sure, there are probably some air borne super micro abrasives (dust, pollen, etc.) that may wear at the wax surface. I would put my money on the fact that it's a black truck that sits in the sun, I bet the surface gets pretty dang hot at times and would soften that wax.
To be clear, from your observations you would say that the wax toppers are gone at this point, but the sealant foundation is still there?
I am eager to start my comparison of the products. Silver car, stored indoors, rinseless washing only.