There are some things to have in account. First, if psychic is uncommon or even quite rare, there would be not too many creatures (or NPCs) with such descriptor in the world, in fact the non-psychic creatures would be a lot more common. This would lead to the fact that psychic powers would overcome spell resistance much more often than spells overcoming psychic resistance; this would mean than being a psychic is a clear advantage.
Second, if there would be a sidebar that indicates things like "outsiders and dragons have psychic resistance equals to their listed spells resistance, undead have psychic resistance equals to their listed spells resistance minus 5 (SR-5)" you would need to have this in account all the time since PS would not be listed in the monster entries existing right now (bestiaries 1, 2, 3, 4 and other stuff), that sidebar would need to be quite simple in order to handle it easily. The OP talks about rules added in the past using sidebars, maybe I don't have a wide knowledge of the rules but are out there sidebars that add (or replace, or invalidate) parts of a big amount of the monster entries existing in the books? please give me an example.
The OP also talks about magic and psychic like well defined (different) concepts, but none of then can be measured in real world, so how can we be sure about magic and psychic being entirely different (or similar)? So to use them as the same thing or different powers is entirely arbitrary, Paizo staff made their choice and we can't say it was right or wrong from a real world perspective.
Regarding fluff vs crunch we could argue that if psychic classes are redundant since they are similar to Wizards and Sorcerers; we can say the same about monks, barbarians, or cavaliers being variations (archetypes) of fighters, druids, rangers, or paladins variations of clerics, and so on.