Passengers with speedbonus require a completly different running costs than mail or fireight. The only way to cure this would be a base plane with all the running costs for the hold; but then freight planes and postal planes had usually anyway different liveries. Furthermore, a base plane without anything will be a constant source of questions ...
Moreover, due to the way the braking and everything else is calculated, all those extensions needs to add a certain weight, so that the total weight is nearly the same. Otherwise max speed and many other stuff will be very different. Or people will be disappointed, that their plane only carries 11t of coal ...