You should not take The Silk Roads too seriously as a book about history. Frankopan is a legitimate historian, but his focus is on the near-east, and that book is essentially an attempt to create a 'universal history' that just happens to center entirely on his personal area of focus. It's not garbage, but many of the key points it makes, and that you cite, are totally off base.

For example, luxury goods did not dominate trade -- bulk goods like grain, wool, and slaves did. So the idea that luxury trade could somehow dominate the history of these whole regions is fanciful -- luxury trade couldn't even dominate trade!

> During this period, the former Roman provinces had very little in the way to offer for trade.

This is not true. They had plenty to offer, from wool to precious metals to grain and more. The slave trade was also not some "Europe -> Middle East" highway: Muslim slaves were also sold into Europe, often by steppe nomads who had more ability to raid those regions.

This is another key flaw in Frankopan's book: he assumes an ultimately colonialist perspective wherein Europe has no choice but to parasitize on the East, having no capacity to produce anything of its own -- a view which can be easily dispelled by recognizing that the literal Silk Road was largely a mutually beneficial trade of goods between two regions that both had things that the other wanted. Wealth flowed both from East to West and from West to East.

> or groups that had founded the city-states of the Italian peninsula, raided for people for slaves

I don't know what you're referring to here -- those people were largely Christian Romans/Italians, and they did not go about habitually enslaving other Christians. The Goths and later Lombards who invaded Italy mostly occupied inland areas, they did not generally found new cities.

> At some point, the raiding groups figured out that it was more productive to force the people on the land to harvest and work the land than to trade them as slaves. The deal became, "we won't take slaves, but in return, you will give us a portion of what you harvest in the land as tribute". Thus, serfdom, taxes, and the idea of rent was born from this.

That is just flatly not how serfdom developed. If you're interested in the post-Roman transitional period, I would suggest Theoderic the Great: King of Goths, Ruler of Romans by Hans-Ulrich Wiemer; some of the later chapters (I believe Chapter 9?) go into detail on the economic development of what later became feudalism/serfdom. In brief though, the coloni who later became serfs were not enslaved by marauding foreigners -- as early as Diocletian, their ability to move from the land was restricted, and in time this evolved (while still totally under the Roman law) into a status of "servi terrae", i.e. slaves of the land. Their landlords, in many/most cases just post-Roman aristocrats, were the ones who squeezed them into a servile state -- not some imagined band of marauders. Indeed, many of the places where feudalism/manoralism was weaker were exactly those which had more Germanic influence -- e.g. Anglo Saxon England, where the Romano-British landlords were largely displaced (and were never that well established anyways), had no concept of feudal relations and had relatively weak restrictions on peasant rights, thus the whole adage about the Normans bringing their system over during the conquest.

I hate to nit-pick, but when you're making historical allegories with the present day and age, I think it's very important to do so based on the actual facts of history.