I'm not arguing with any of your points. However, many of them involved the poor people leaving the metropolis to live somewhere else. They also involved said poor people being used in the war machine to build more empire, and hold on to the empire it already held. The poor people still living in Rome did benefit, but this was mostly a form of crowd control. The rich people benefited vastly more from empire than the poor. And when the empire collapsed, Rome was left as a shell of its former self for centuries. Not to mention the fact that the importance of Rome as a capital waned during the empire's last couple of centuries.
Are you telling me that the people living in slums in Victorian London, toiling away in the workshops, shipyards etc. paid a pittance, with a lower life expectancy (for a long time) than people still in the countryside, benefited from the British Empire to the same extent as the British elites did? (Mind you, London has survived (and thrived) since the end of the British Empire.)
Veii is just next door to Rome! How could settling it, and incorporating it to Rome, be seen as leaving the metropolis! If you ask the poor plebians being settled in Veii in their own plot of land just a few miles across the Tiber if they're actually leaving Rome, they'll laugh and say no.
It doesn't matter how much vaster the rich benefited from the empire more than the poor. It only matters that the poor also benefited, and thus, their lives were better before the acquisition of empire. Thus, ordinary Roman plebs were vaster better off than they were before Rome's acquisition of empire. It's not a game of relative benefits, but of absolute benefits, and thus, both rich and poor benefited.
And yeah, so what if the poor people were soldiers? Those people gain a lot from fighting and conquering. Loot, plunder from defeated enemies, the enemies' land as farms for their landless kin, salaries, etc. Those were no mean things in the Ancient World, and it vastly exceeds their ordinary income in peace. And it was an honor to fight for Rome during the Republic, and during the empire.
And Rome and Italy are the metropolis. People leaving Rome and settling in other parts of Italy aren't really leaving the metropolis. At first, Rome was only the metropolis, but by the first century B.C., all of Italy could be considered such.
So what if it is crowd control? They still benefited from the empire. They got free food, free entertainment, a better standard of living, etc. And that would not be possible without the empire giving free things to Roman Citizens. Our welfare system today would might as well be labeled crowd control to prevent people from rioting and dying.
As for what Rome was after the empire collapsed, that doesn't matter to the Roman poor and plebs when empire first came and during it's height, since that are we are talking about.
What matters for this discussion is that when it could, the Roman Republic and Empire gave lots of benefits and services to the poor plebians, and it was real and they felt it as real too.
Are you still insisting that the poor Romans didn't benefit from Empire? Because they most certainly did!
As for Victorian London, I'm going to leave that to other people more knowledgeable to me. But even if your point on London stands, the thing I'm trying to disprove is that all empire tend to not benefit the metropolis at all. One counterexample would disprove that, and show that, while it might be true in some cases, it is not in other cases.