IQ has been crashing since about 1850. As I've said before, aristocrats have a near-replacement fertility at the best of times, while the lowest slaves are all r-strategists and breed like rabbits. Almost exactly like rabbits, in fact.
However, we've only regressed to like the 1600s or 1500s.
Falling IQ cannot explain the bulk of the degeneration.
Especially since you can go look at the Moscow Metro. Don't have to go back hundreds of years to find something less Fascist.
Child mortality has dropped from 50% to 1%, but aristocrats had hardly any child mortality to begin with, so that's nearly all extra underclass children. Massive explosion in followers while leader populations haven't budged except for peasant=>aristo atavism and de novo mutations.
However, it's not like leader populations have actually zeroed out. Good taste hasn't vanished from the Earth...despite all efforts to make you believe the contrary.
Rather, it is being suppressed. E.g. the writer's strike revealed that hollywood's writers are transcendentally better at writing than hollywood would have you believe. Executive meddling? Shouldn't one executive break ranks? What's stopping one of these writers from going "fuck it" and writing for a web novel site instead?
In reality, even the writers actively hate glorious art. They cooperate with their own (our own) immiseration.