The Romans had by the time of the official "fall" of the Western empire already introduced the practice of stationing Germanic mercenary cavalry directly with landowners and towns in Italy, so that the mercenaries could collect their own wages directly as taxes. This feudal societal organization would basically remain unchanged in the Ostrogothic and Lombard kingdoms. Landowners and mercenaries had a shared interest in preventing the peasants/commoners under their control from leaving if that impacted income, and would be definitely capable of hunting them down if they did.

I do not have the impression, if you look at the sources covering later attempts by the Eastern Roman empire to expand their influence in Italy at the expense of the barbarians, that they had much popular support for doing so. On the contrary: small Lombard feudal armies for instance regularly defeated larger but very low morale locally sourced (Eastern) Roman armies. And parents complained about their children dressing as barbarians to look cool. That doesn't give the impression that the average former Roman was willing to risk his life to be able to live under an emperor in Italy. At best you could describe it as an attitude of apathy.

Do keep in mind that horses are difficult animals to feed compared to most livestock. And even more difficult to breed if they don't live in a large group, and you don't understand the process of artificially inseminating them. Horses are finicky about that. Just owning a stallion and mare is usually not good enough.

In areas with a lot of suitable pasture horses can be common, while in densely populated areas with no natural grass they are a luxury imported from abroad, difficult to keep alive over the winter.

Research on age and gender distributions of horse bones collections in the Roman empire and China shows that these cultures mostly imported their horses and breeding was relatively rare. Steppe peoples on the other hand ate and sold young stallions in great quantities.

In the relatively wet plains of Northern and Eastern Europe plowing with horses was common in the middle ages. In the drier landscapes of the mediteranean oxes were used for that purpose.

The Romans describe Germans as people who price keeping cattle over doing agriculture and have the annoying habit of burning down their own villages, granaries and fields and retreating into marshes or forests with their cattle if attacked. Feeding your army and ambushes will a big problem, and there is little loot to be expected.

Interesting detail. For centuries, European ship cannons were perceived to outrange Japanese and Indian cannons because the Europeans were always a step ahead on daring to reduce windage, because of trust in their production processes.

There was a topic half a year ago or something about 'ugly' late Roman coin faces vs. 'pretty' early Roman coin faces. The explanation people settled on was: mass production. Heavy pure gold and silver coins made in one production line for a relatively small upperclass is something else than making thin coins from alloys that don't flow well in many production lines for millions of users. So to keep the coin faces consistent and recognizable you made them simpler. The problem of scaling up production while keeping consistent quality is as old as civilization. And it still is, obviously, in for instance nanolithography.

If supplying fuel for a labor-saving machine (for instance a sawmill) takes just as much or more slave labor as is saved by the machine (sawing), it is prefereable to just put the slaves directly to sawing instead of gathering fuel because it requires less oversight over the slaves.

In roman times the most efficient to move a large amount of fuel would have been a trireme with slaves at the oars and a very basic sail. In the 18th century the same would have been possible with a ship manned with just a handful of sailors. From this perspective the wind power revolution in the age of sail contributes to the preconditions for the industrial revolution.

Every cannonball needs to fit the barrel as closely as possible to maximize the power of the cannon. This leads to two big technical problems: firstly precision, and secondly metallurgy to keep the cannon from exploding when you achieve the desired tight fit. Precisely the two key problems you need to solve for interchangeable machine parts.

The Japanese successfully fast-tracked themselves for Industrial Revolution with a decades-long program trying to replicate the range of European cannons. And this was with access to European scientific and technical knowledge through the Dutch trading post (Deshima) in Nagasaki. Otherwise it would have been even more difficult.

I wonder what would happen if you carpet bomb the Russian-occupied part of Ukraine with a trillion in Ruble bank notes.

I don't think anyone claims there was no migration, just that there appears to be no definite identifiable point at which a mass migration or large scale invasion happened. Michael Pye's The Edge of the World is an interesting read relating to this topic. "Frisian" trade networks spanning the North Sea coasts may be a big part of the (hypothetical) answer. There are lots of references to the Frisian sea, Frisian trading posts on all North Sea coasts, and Procopius (from a vantage point in Byzantium) writes that Brittia is inhabited by Angles, Frisians, and Brits.

"Frisian" as used by Latin writers in the Dark Ages should be understood as a purely geographic label: the oldest written history of the Counts of Holland mentions that the Low Countries are inhabited by Saxons, but are traditionally called Frisians by the Romans and Franks (after a tribe that used to live there in Caesar's time that gave the area its Latin name). So for practical purposes Frisians = Saxons. One is an endonym, and the other an exonym for ethnically the same people in a specific area.

Because transport over land was much slower, harder, and more dangerous than transport over sea, this trade network would have had a major impact on linguistic transmission (creating perhaps a sort of creole Lingua Franca of the markets on the coast), and migration from coast to coast was simply a matter of individuals, families, small bands of adventurers, or small villages booking passage over the course of centuries. If migration happened this way, the migrants would easily all pick up the same language (closely related to their own). And diplomats traveling between the kingdoms in England would have depended on that same trade network for their travels. Travel itineraries of missionaries for instance do suggest that hopping from port to port on the North Sea was greatly preferred over inland travels.

Kingdoms that formed may have picked this trade language as their official language merely as a matter of convenience. It was the language of wealth and power, and of interaction with the other kingdoms and foreign bands of cheap mercenaries. Like the US picked English, over for instance Dutch, French, Spanish, or Navajo, etc. Including some kingdoms that were, by historical accident, dominated by clans of Angles.

This account leaves open the question of what happened with the trade network if it was already so well-developed. How did English become isolated from the mainland Germanic languages? Another hypothetical: The Franks caused it to collapse when they conquered the Frisian and adjacent Saxon kingdoms on the mainland, temporarily isolating Britannia and the Scandinavian coasts from access to the trade network, and making it give way to an era of North Sea raiding (the "Viking" era) that pushed people away from the coasts and increasingly turned inland travel into the preferred method, and reduced shipping to short distance crossings of the English Channel.