I think the big issue was not that the foederati were specifically outside the normal Roman political and military world. A German general or nobleman who took

I think the big issue was not that the foederati were specifically outside the normal Roman political and military world. A German general or nobleman who took over a fragment of the empire would not have caused a major change to the political order and their territory would be easier to re-incorporate into the empire. What I'm getting at was why were the Franks taking over Gaul different from the Gallic Empire. My impression is that the difference is that there was a German political/military order that replaced the top layer of the Roman political order (although the bottom layers which included the city administrations and the Church continued to function), which included fragmentation of authority after the first few rulers, and that meant that the old administration could not be reincorporated into a renewed Roman Empire in one go. (Of course, this re-incorporation of a German breakaway kingdom did happen in the case of Italy which was briefly reunited with the Byzantines, my explanation doesn't really account for that, it would be interesting to examine how difficult or easy it was for imperial administration to resume (ie, was it treated as a military frontier province or did the old Italian administration get reconstituted)) And I think this is because the foederati system helped preserve a separate German political order even after the tribes were fighting for the empire and living within the empire. The alternative I can think of was if the king of the Germanic tribes settled in the empire was given a formal governor position, or some other position within the Roman hierarchy. My understanding is that that generally did not happen, although I could be wrong.