Almost by definition, empires are multi-ethnic.
Empires are won and held together with a mixture of force, guile, bribery, etc. They are by no means a liberal nation-state and tolerate no dissent or action deleterious to governance.
As such, SS's point has no relevance to the liberal nation-state. It does, however, have relevance to the failed USSR, Austro-Hungarian Empire, and other similar empires.
Roman citizenship and suffrage was "liberalized" over time. As more & more non-ethnic Romans became citizens, the Roman state became less & less stable. I am not arguing causation, but my point is that the thesis, "A multi-ethnic and multi-lingual state with universal citizenship/suffrage can be stable" is not supported by any period of Roman history.
The Roman Republic & Empire was not particularly innovative, relative to the creative chaos of the Hellenic world. Its art and technology during its heyday was moribund relative to the Greeks in their heyday. Thing was, Romans knew how to act & execute in ways the Greeks could not.
The revisionist side of "Late Antiquity" is grasping at straws. Yes, there was some progress. Without it, Europe wold never have gone on the the less-dark Middle Ages & Renaissance. The record shows some serious retardation in nearly every sphere of human endeavor and a cataclysmic fall from Roman civilization. I am not sure how pointing out a few high points discounts the record of overall decline.