For whom was the decline and fall of the western Roman Empire that commenced with the Antonine Plague a decline and fall?
It was a decline and fall for:
Those at the top of the state apparatus who saw their resources collapse, and their ability to avoid barbarian power projection into the empire decline.
Those who were rich.
Those who were urban, dependent on the flow of resources from the countryside to the city.
Those in the countryside who had been prosperous and free, and who were now enserfed.
Those who benefited from large-scale efficient production and distribution of conveniences.
Those for whom the maintenance of the pax Romana was really important.
The question is: what happened to those on the bottom in the countryside? They now are much more productive. Yet they face increased pressures from above to exploit them more in order to compensate for the reduced rent flow from depopulation, and they suffer from increased dissipative violence—both from their superiors and from barbarian invasions. Yet on the other hand their increased value might have given them increased leverage: that was the case for western European serfs in the aftermath of the Black Death.
My view: slave —> serf a definite improvement. For everyone else, a decline: loss of conveniences with the end of large-scale efficient production, loss of resource flow to city-dwellers, loss of resources that had underpinned the pax Romana. What is uncertain is the relative numbers of the winners and losers—and whether even the winners in terms of consumption were better off facing the barbarian rather than the slavemaster.
In any event, very interesting: Willem Jongman (2006): Gibbon Was Right: The Decline and Fall of The Roman Economy: "For the naive historian, it would seem that we now have all we need: we have a range of examples of catastrophic decline, and some potential causes. What we do not yet have, however, are the mechanisms by which this shock propagated through the economic and social system. Imagine a pre-industrial and largely agricultural economy in a fairly stable equilibrium. Next that equilibrium is disturbed by mortality.... The biggest economic and social change, however, was to the land-labour ratio...