"The fact that the classical world eventually was replaced by modern nonsense should not be held against it."
How about instead extremely the opposite of that?
You may have noticed I'm none too fond of Christianity, but I would forgive all its sins were it effective at suppressing the modern heresies. You had literal anti-blasphemy laws and you still couldn't do it! Pathetic. If instead it had worked, I would be over here scratching my head saying, "Well, I don't get it, but they obviously know something I don't."
The fact it was replaced by modern nonsense is exactly the classics' highest crime.
Though more accurately, modernity isn't modern, relative to the classical world. We are Fascist, they were Fascist. They were less Fascist due to having a proper lord ratio, but the latter wasn't good enough to paper over the former.
But basically, they killed Socrates. They couldn't even tolerate being near the truth. They had to symbolically eradicate it. Their core value was raw hatred for Reality.
If there were some sudden disaster modernity could be traced to, such as compulsory education or the black death, then the classics could perhaps be excused. Even the most anti-fragile system dies if you hit it hard enough. Instead it's been a long, slow, and constant 900 year decline. All failure, no victory. Not anti-fragile, just fragile.
Christianity didn't declare war on these heresies, it adopted them. It adopted them because Christianity was too weak to notice Sophism intentionally hacking its adoption reflexes. Imagine the Soviet Union appeared and the Christian races were like, "Yeah, no," and shut that shit down. Instead feminism is actually 1100-vintage courtly love, slightly intensified and stripped of any decoration.
It is likely that Christianity was always heresy. That Christianity was itself designed through Sophistry to hack Roman social vulnerabilities. Constantine attempting to adopt it should have triggered an existential war, because you don't follow suicidal rulers. Not that I terribly disapprove of Rome killing itself. They earned it.