> That the Roman Empire was to some extent decentralized doesn't matter though - they had all of the power and were able to extract all of the surpluses from their vassal states.

Yes it matter, because it means you can't treat it as a singular entity. It never acted as one. The success of an emperor depended to a great deal on squeezing just hard enough to not give anyone designs on thinking they could do entirely as they pleased, yet not so hard it'd trigger rebellions. There was no capability to prevent all kinds of slack in the system in every layer.

They had "absolute direct power" as long as it was only ever exercised with restraint, or in limited areas.

> 1) Since when did the Roman Emperors or the Feudal Nobility have any responsibility?

Read the rest of what I wrote. The point is not that there was an explicit, stated responsibility, but that it is inherent in that those who mismanaged their workforce faced a declining workforce. You point out yourself that a large proportion of Rome's inhabitants were actual property. Let them die, and the value of your property and the profits you can extract from it goes down. You had no ready source of replacement. A feudal lord facing increasing demands for tributes could not meet it if he let his labour force die, because it's not like he could take out an ad and have a flood of new workers arrive.

That is the relationship that capitalism removed. That does not mean the feudal system was good. It was awful, and capitalism was a distinct improvement. But it means a fundamental change in the extent to which the parties could afford to attempt to extract maximal value from their workers at all cost, and that was a key driver in making capitalism viable until further automation and expansion into new markets made it less worthwhile to fight to retain that pressure. But the dissolution of those bonds remain, and it means the dynamics of how capitalism deal with growth are very different - for both good and bad.

> 2) Again the Adam Smith: you're not counting surpluses. Economies create vast amounts of consumer surpluses. Whatever the capitalists are able to hoard away in their bank accounts is small in comparison to the value created by consumer surpluses.

Surpluses are only relevant as long as you have money to access them. As such, talking of surpluses is irrelevant without addressing what happens to pressure on employment and wages once growth hits a wall. This is the main point of Marx criticism of capitalism: That capitalism does create vast amount of additional value - he was a massive fan, expressing absolute awe at the possibilities capitalism created - but the inherent competitive pressure in capitalism means that capitalism will eventually start to eat away at the need for labour. That's a good thing for society in the long run. But in the short run, it means addressing how to ensure access.

> It's not a paradox that people are immensely more wealthy in capitalism despite however 'unequal' the distribution of accumulated wealth is.

You're right, it's not, because capitalism still has room for growth, during which it has no incentive to squeeze everything it can out of efficiency improvements.

It has no relevance, however, to the question of what happens once further growth is impossible and competitive pressures have to turn inwards and focus on eliminating labor to squeeze out ever thinner margins (remember, Adam Smith also argued that profits were "always highest in the countries which are going fastest to ruin"; high margins are a sign of inefficient competition)