If the Vikings in Essex had a well-developed liberal theory, for example, we'd have no idea of it, because none of their writings survive. (There were lots of slave graves, though, so they probably didn't put it into practice any more than Jefferson did, if they did have it.) There are some less depressing archaeological finds like Çatal Höyük, whose setup suggests hunter-gatherer-like egalitarianism, but we really have no idea what their philosophy was or what kinds of freedoms they enjoyed or valued; by the time cuneiform was invented, Çatal Höyük had been abandoned for 2500 years, after 2200 years of prosperity.
But there's really nothing resembling liberalism in classical Rome or Greece or medieval Islam or ancient Egypt or the Vedas, and it's quite a stretch to read liberalism into the Mozi or the Laozi.
I don't think it's such a sudden collapse, though. Liberalism has always been just one ideological current among many in the US, and there has never been a time when the US fully lived up to its ideals. Indentured servitude, chattel slavery, the Trail of Tears, the Comstock Act, Prohibition, imprisoning people for pacifism, the Indian schools, J. Edgar Hoover's deep state, the draft, Jim Crow, the imperialist conquest of the Philippines, the Chinese Exclusion Act, mass imprisonment, the Sonny Bono copyright extension act, the plea-bargaining system, Obama's mass deportations, the Comics Code, civil forfeiture, the FCC granting local media oligopolies, and the MPAA ratings system are all incompatible with liberalism, as I see it.
But I do think the last 25 years have seen a decisive turn to the Right in this sense; the main political debate in the US is no longer between liberalism and conservatism, but between progressivism and conservatism, both of which are opposed to liberalism.
In a liberal society, people can find information about birth control, move to the city, and escape bad marriages, three factors that seem to substantially cut down on procreation. Historically, liberalization has been closely associated with reductions in birth rates, though not exclusively so (Iran and PRC being two illiberal countries that achieved low birth rates recently.)
And it's one where you can go BASE jumping, try to invent new kinds of flying machine, put a pulsejet on your bicycle, improvise your own shotgun, or take the medicines that you think will be best for you, all of which are pretty bad for survival if you make a lot of bad decisions. Like me.
Moreover, it's not clear that liberalism is compatible with democracy; democracy might inevitably succumb to a Pisistratus or a Robespierre, which certainly isn't liberal. Ultimately, i think, liberalism cannot survive the existence of unlimited government power, even if it is democratic. Sooner or later it's two wolves and a sheep voting on dinner. (Though, as the handling of the covid epidemic in the US demonstrates, sometimes all three vote to huff spray paint for dinner.) The Mozi begins:
> To enter upon rulership of a country but not preserve its scholars will result in the downfall of the country. To see the worthy but not hasten to them will make the country's ruler less able to perform his duties. To the unworthy is due no attention. The ignorant should remain without inclusion in the state's affairs. To impede the virtuous and neglect the scholarly and still maintain the survival of the state has yet to be, indeed.
Finally, most of idiocy is non-genetic. Idiots have always outnumbered the smart and always will, because humans are just sort of inherently pretty idiotic. So it doesn't matter that much who's doing the procreating.
Either they pandered to the masses or realized that there's no way a liberal society with an uneven mix of tolerant and intolerant people can survive.
I like to hope the future of humanity is good, I help individuals, but I would really not mind if millions die in a nuclear fire.
Including me, of course - if anyone wants, you can kill me anytime. Alternatively, I have some ideas about a certain type of drone.