I mean he was right (in that there was a decline of the West, or Europe in particular - I am not familiar with the content of his book beyond that). The West went from conquering & dominating almost every place of the planet despite being but a small minority of its population to losing the vast majority of their conquests. The tech difference between say Paris, 1918 & Shenzhen, 1918 was much higher than today - in fact I would say Shenzhen is ahead.