But different conditions hold today. The Gothic armies were virtually identical to the armies of the earlier Celts/Gauls who the Romans had crushed; even the Magyars (~1500's CE) used more or less the same tactics and organization as the Cimmerians (~ 700 BCE), though they did have stirrups, solid saddle trees, and stiff-tipped composite bows. Similarly, IIRC, the Roman armies didn't make use of any major recent technological innovations. This no longer holds today; the idea of an army using technology hundreds of years old being a serious military threat to any modern nation is frankly ludicrous. Technological and scientific development has become much, much more important than it was during Roman times.
(And, btw, it's not really accurate to say that, in practice, the barbarians were all that much much worse than the Romans in terms of development and innovation; technological development in Europe didn't really slow down all that much during the Dark Ages and the Romans had very few scientific (as opposed to engineering) advances anyways- most of their scientific knowledge (not to mention their mythology, art, architecture, etc.) was borrowed from the Greeks.)
Yes, but the culture of enlightenment and innovation within Greek and Roman culture had already been falling apart from within. The culture of Classical Antiquity was outcompeted by less enlightened memes.
http://vimeo.com/22099396
What do people think of this, from a Bayesian perspective?
It is a talk given to the Oxford Transhumanists. Their previous speaker was Eliezer Yudkowsky. Audio version and past talks here: http://groupspaces.com/oxfordtranshumanists/pages/past-talks