I doubt the Britannic tribes will manage to do that, I just don't really see that happening though I may be wrong.While you are correct to say that the tribal confederations on the European borders never formed states, the Franks, Vandals, Visigoths, Suebi, Ostrogoths and Saxons would argue that if given the chance, a tribal confederation could move in on the weakened empire and take advantage to settle inside the empire and form semi-functional states.