I am probably one of the few people on this forum who was actually involved in truck design (Bedford) albeit many years ago. It is now substantially controlled by accountants and the design engineers do what they can within tight fiscal limits.
Disc brakes, for example, were initially fitted because they were cheaper. That they faded less and worked after river crossings was a bonus.
Vehicle (and tyre) design is not like bridge design (where you must have a substantial safety factor). If yours was designed to carry (and stop) 1 tonne, then carry up to that. If you want to carry 1.5 tonne, buy a bigger truck.