I think that's because, in this hypothetical scenario, the force of "evil" continues to exist. That is, in in the absence of Force of Good, the triumphant Force of Evil pushes psychopaths to burn down orphanages, etc. and drives regular folk into turncoats or apathy and despair.
Basically. PS starts with the conceit that good exists because people believe it exists and if a villain (or PC perhaps!) started changing that belief, you might have a scene in, say, Arcadia, where this iconoclast convinces the people of Mount Clangeddin that there is no such thing as "good", there is only this phenomena of "peace" and that comes out of "order" and then the dwarves, convinced due to the creature's actions, spread out over the planes to help convert others, and maybe Silverbeard goes to talk to the other dwarven deities about this remarkable individual with these interesting ideas and through some other effort the entire dwarven pantheon is convinced and now things start rolling because every dwarf on every world slowly starts to agree that LG is an illusion of ego and there is only truly Law, and influence spreads...and on and on. As this influence grows, layers and planes start becoming part of other places - Mount Clangeddin becomes a gear on Mechanus and soon the dwarven heavens join it and dwarf-bots suffuse the planes and the Slaad become nervous and the story goes on and the assuming it's the antagonist doing this the climax sees the party in the plane of Elysium as Law and Chaos try to claim dibs on it and the guardinals are fighting a civil war and they must convince the Last Good Soul (perhaps the spirit of a child) to somehow remind everyone that there is more than Law and Chaos and Evil, and either they succeed and Good gets (gradually) restored or they fail and there are no upper planes.
...but of course there are still people helping little old ladies across the street, probably, the consensus just views this as their duty to the social order, rather than as "good."
And that's just one interpretation - other groups would probably have their own.
What if the question was: if the forces called "good" and "evil" (and "neutrality") ceased to exist, what would actually change?
Couldn't the outer planes and angels and demons continue to exist, founded on individual virtues and sins and beliefs that continue to persevere in the hearts and minds of the immortals and mortals? If people still cherish their values, must the outer planes come crashing down in this new secular world?
Based on the PS idea of the planes being
made of this belief-mana, the obvious question would be: what do people believe in when they don't believe in the alignments? I could see a story based on the idea of the outer planes collapsing entirely (perhaps motivated by empiricist-Sensates in league with elementals - believe in nothing beyond facts!), or one simply based on them transforming dramatically (what does the consensus believe in now? does a monotheism perhaps change the consensus to believing in a dualistic multiverse? is good/evil less relevant than death/life or honor/dishonor? what changes?), but the end result wouldn't be the Great Wheel, and those that are at risk of being believed out of existence would certainly fight against that! Whether those are your allies or your enemies would probably depend upon which side of the iconoclasm the party is on.
Again, the interpretation would vary between tables. PS sort of wants you to figure this out for yourselves.
