You must spend alot of money on the best pad but that pad offers no support to spreading out the load that you butt puts on the horse's back.
Pick up a treeless saddle and bend it front to back. It has no support whatever. Trees were invented to act as a foundation of support, all good houses, cars, everything starts with a good foundation and builds from there. You guys are taking a step backwards.
PSI is ultra important. It is what allows the horse to carry weight, put even 3 psi on a horses back and you are going to sore him.
Example. Put a 100 rock on your shoulder and carry it for any length of time?? Now take a 100 sack of feathers and try carrying if for the same length of time.
It is about stability, support and spreading the load.