In fairness the Ronan Legion was superior in the field from 275 BC through its abandonment after Adrianople. (378 AD).. The reason it was abandoned was that it was too costly to maintain.
Calvary also was becoming more important as well, though you really needed both. smaller polities made it difficult to field a large army that included both.
As it was the Roman polity embracing the Mediterranean held sway for a four century span plus the era of its rise of three centuries. After the fall in the fifth century of the Western portion, the Eastern portion still dominated through even the fifteenth century but cretainly an echo of what had been
It provided a blocking force allowing all the successor states in europe to fully emerge.