That is a good question to which I do not have a sensible answer other than a vague "both, probably". I suspect empires follow more or less the same trajectory

That is a good question to which I do not have a sensible answer other than a vague "both, probably". I suspect empires follow more or less the same trajectory as many family fortunes do: the founders build a base upon which subsequent enterprising generations build an empire. Once the empire has been established and more and more generations grow up with the certainty of it being there decay sets in. Eventually the empire either falls prey to its neighbours, decays without outside intervention or enters a steady state. The steady state makes it vulnerable to new ideas which might not necessarily be better than current ones but at least they are new or progressive.