Liberals see progress as like climbing a mountain. You climb past certain obstacles, but others overcome you so you slide back down a bit. Gravity in that analogy represents the pull of the natural default social order (animality, the dominance hierarchy). It's our animal nature that pulls us back and that sabotages our modern humanistic experiments.
So I'd say it's not a case of decline so much as a failed experiment, a short-lived burst of humanistic creativity. The question is whether there's a sustainable, dignified humanistic alternative to conservative animalism.
Yes, that idea (but not the wording) comes from Dostoevsky's Brothers Karamazov.