Thousands of years of history have been forgotten. The culture and civilisation of peoples like the Sogdians and Turks was once greater than that of the peripheral civilisations. The cruelty of the Central Asians has likewise been exaggerated. In most cases of war with the peripheral cultures the evidence clearly shows the peripheral empires were the aggressors. Even where that is not the case the Central Asian empire builders never committed any cruelty that was not routine for the peripheral empires too. Cities were sacked and enslaved but the Chinese and Romans did the same thing.
Beckwith ends with a long rant against modernity. He identifies this as ideological extremism (whatever the particular ideology). I agree with his analysis but I don't think it is an essentially intellectual error. I believe it springs from a combination of mass literacy and centralised military power. Ancient empires sometimes had centralised military power but their capacity for mass propaganda was limited. With the advent of newspapers and then television it became practical to push a single pretext to justify the power of the ruling class. The pretext can be Christianity, Democracy, Communism, Islamism etc. The essential element of this ideology is the military power that backs it up. In every society there are plenty of people who recognise the ruling ideology as a lie, especially when it starts. This opposition is cowed by threats of brute force until it recedes. When the ruling class fails the new rulers promulgate a new ideology to justify their power.
The real beauty of this book is the incessant flow of people and ideas across Eurasia over thousands of years. It is clear that ideas flowed freely (if slowly by our standards) across empires. China and Europe exchanged many ideas indirectly.