His result is that this is not true. The trope is also related to historically untenable ideas such as the "noble savage" or, if you take it too far, fascism's quest to bring back "real men".
Acoup tought me a lot and changed my mind a few times, and the paper is interesting but far from the state of the art. Even so, I don't think their disagreement is big, and looking at the core of a new empire as barbarians/ noble savages is an incorrect label of the old empire.
I'd say the power of a group of people depends on not only the size, but also on the morale and organization. Mr Deveraux mentions something similar at the end of your link.
Looking trough this lens, the change in Age from Intellect to Decadence is a hidden breakdown of the organization. Even if the number of people rises, the coherence and trust of the group disappears.
The other end of the Glubb paper seems similar. The old empire is less organized than everyone expected, and the 'barbarians' are actually a new group, better organized and more willing to adapt then expected.
His result is that this is not true. The trope is also related to historically untenable ideas such as the "noble savage" or, if you take it too far, fascism's quest to bring back "real men".