In any case, remember my focus on morbidity vs. mortality? It might be apropos in this case. The uncertainty and political instabilities due to the collapse of the Pax Romana could plausibly have increased mortality as peasants were exposed to the erratic depredations of barbarian warrior bands. But as depopulation occurred, in part because of withdrawal from the frontiers in places like Gaul (France) an western Germany of most farmers, those who opted to remain and take on the risks would be relieved of some Malthusian pressures. I think the chart of European heights does point to this as well, you can discern a slight upward trend after the Black Death due to a radical population reduction. I've reedited one of the charts for clarity:



As for Barbarians to Angels, the author doesn't really make me reconsider. I've talked about my skepticism of the idea of revisionism in regards to the decline of Rome. The author argues that technological advances occurred during the Dark Ages, and that many cities remained active nodes in trade networks. But the author's treatment is highly qualitative where he had concrete examples of how complex society persisted after the collapse of the Pax Romana, and he repeatedly scolds the readers to not judge Dark Age societies by modern standards which would tend to align more with the priorities of Roman civilization (e.g., reading, writing, arithmetic, public architecture, basically what we might term civilization). If the author wants to strip the term "civilization" of any normative biases brought to bear due to the prejudice of moderns, the argument is won, amassing a large collection of ornate weapons with which one might be buried is just as Cultured as writing letters to your friends laced with literary references. A good cup of mead is at the same level as a Falernian.