But...why should it be used in an affirmative sense?
I didn't say it should be. I said "if we want to...".
If you look at it with something like a Nitzschean "pathos of distance" (I hope this is the right term in English), blind for the suffering of the masses, of course you could make an argument that the late 19th and early 20th century was a peak of art and culture in some sense.
I have to add though that personally I'm not enough of an expert on art and literature to make such a statement.
1900 makes sense because "Westerners" (the Europeans) or their progeny (the US and the Republics of Latin America) dominated or heavily influenced the entire globe. Said grip has seriously loosened. I don't see why democracy, women's rights, pluralism etc. are more fundamental to the Western experience than colonial imperialism and military dominance.
If you look at it that way, I absolutely agree with you. Thinking about it, your definition of Western Civilization is a lot more to the point than mine