When did the Roman Empire fall (your thoughts)?

[Robert Vermaat]

IMO it says much about the paucity of regular Roman troops that Huns were necessary not only to get Aetius back in power after his defeat in 432 but to deal with burgundians and visigoths later.

Not at all. Ceasar and Pompey raised their own legions, Aetius could not do that, but he could hire Huns. Or Goths or whatever. The reason - political power - is the same. It tells us nothing about the strenght of the Roman units.

The difference is that, in the good old days, foreign units augmented Roman strength, whereas in the fifth century they seem to have largely replaced it.

Now you're argumenting in circles. You've already determined for yourself that the fifth century only knew weak Roman forces, so your interpretation of the mercenaries is that they were hired "evidently because the regular forces no longer amounted to much". As I made clear in my example of Ceasar, the hiring of mercenaries tells us nothing in theory of the strength of an army.

Elton argued that the quality remained OK but "not enough soldiers" was part of the problem.

Why do you think Ceasar or Contsantine, or any Roman emperor really, needed mercenaries - to get extra soldiers! But like I said: that, in itself, does not tell us anything about the Roman units.