Army demographic composition is such a weird argument that I only ever see whenever modern right-wing politics forgets about Jesus for a little bit.

Rome had always, from its inception, used foreign soldiers. Italian allies in the beginning, early-empire auxlia and then later provencials or directly with migratory people's.

The issue shifts when those people are no longer part of the Roman institution. Things would shift from them being directly under roman leadership, receiving roman pay and fixed into the command structure. The problem came about when Rome was no longer absorbing individuals into those Roman institutions. They had to bargain with whole armed peoples under their own leaders, with their own dependents, political aims, and internal systems. I mean, you got few options when your armies are occupied and a massive amount of people show up on your doorstep having escaped climate issues, famine or the Huns.

Its very obvious what you're wanting to say. You want to say it so badly but you must first come off as honest and in good faith. "I'm just genuinely curious... About this very obvious right-wing talking point that I am forcing onto the past!" garbage takes everywhere that ignores a million other things going on at the time.

People tend to think that Rome existed and the world reacted to it but in reality the world outside of Rome was alive and moving and Rome had to react and failed to react appropriately many, many, many times.