You know "the age of plate" as you call it lies 12 centuries ahead of the roman empire from the first to the third century yes? The so called barbarians used lamellar armor hundred few years before the roman empire, especially the Alemmanic tribes who battled the west roman empire. Also they used short swords like Sax (which evolved into Langes Messer) and spears, its not called the Iron Age for nothing.
Actually romans did use plate armours but yeah, i meant the 15 century stuff
. Armours so good that you could forget the shield and use some big 2h weapon effectively.
Also where did i say that romans invented armours and swords? Actually their lorica hamata is probably celtic stuff. Anyway "barbarians" did have only few armours, most of the people couldn't afford one. Instead all romans had good armour after the marian reform (the state provided good armours even for the poorest soldier).
Short swords were used not only by roman of course, and i can't think of one people in classical times that didn't use spears.
Its always about money who wins a war.
Economical might = victory.
Rome got sloppy economically, became broke and died.
Of course wealth is very important, but sometimes who had the most $ didn't win. Vietnam War, Greeks and macedonians beating persians multiple times, mongols conquering the chineses (and then most of the world) afghan resistance against british and russians for example.
There are other important factors, like will to fight, technology,training, tactics, weakness to some unknown disease that the enemy could bring, etc etc.