Magic resistance won't crash a civilisation.
Spellcasters have two aspects:
Weapon of Mass Destruction
Utopia Enabler
Magic resistance only tempers the destructive aspect, resulting in a world gone tippyverse.
Widespread knowledge of magical power results in more people with power. Magic can be used for mass destruction. So you're literally talking about a world in which lots of people are walking around with weapons of mass destruction.
Fixed.
Yeah but he mentioned he didn't want it to be about "people are jerks", which is what it boils down to. I figured I could turn it into more, the world would be split in utopian/dystopian portions, depending on who the local strongman is, magic as a tool for conquest is fast and immediate, but its not so hot for holding territories so much as mutually assured destruction, whereas, without the XP nor spells known component for crafting(because PF allows crafting of wondrous items without the prereqs, it just takes a skill check at a higher DC), the more utopian concepts can be realized with large numbers of weak casters.
So you advance the whole shebang a few centuries and magic does everything in the civilized bastions, while in darklordpolis all non magi are crushed under the bootheels of their superiors. When you turn the magic OFF, the utopians crash and burn, nobody knows how to do anything without magic. The dark lords are overthrown, but their peasants are likely ignorant of just about anything as a control measure.
So you have a society with:
A) dispossessed mages, the ones making the world turn can no longer do it.
B) falling crowns, the darklords will face their revolts
C) failing facilities. Just imagine a modern city without electricity or oil.
D) lost knowledge. The peasants will have concepts of how to eke out a living from dirt, but no technology whatsoever. The utopians will have a lot of art, philosophy and magic theory, but the intermediate, PRACTICAL arts are for the most part lost.
Besides, the Locate City Bomb doesn't exist in pure PF(rather than PF/3.5 hybrid) yet. The components aren't present, so you'd have to settle for spawnpocalypse.