When the separation came it was more of a civil war between two halves of an Empire struggling for superiority than armies of loyal Celts supporting the Central Empire.
There is even a possibility that in the Western Empire the Celts were on the cust of success and that the barbarian invasions were deliberately encouraged by Rome and/ or Byzantium to destroy Celtic independance and replace the Celts with peoples of a different culture.
The mystery of the”Year of Migrations” then takes on a vastly different meaning; that of a massive genocidal attack on celtic society.