I wouldn't say that righting the course of Roman-German and Roman-Goth relations with major defeats constitutes failure. They never did get Germany, and that halted their expansions northward and hampered them eastward as well. And Adrianople cut down the flower of Roman nobility and forced Byzantium on a course of long, ruinous payouts instead of smackdowns.
So yeah. Those guys.
Mehmet was the cleanup guy.
Neither one of those things is true.
Arminius' victory in the Teutoberger Wald destroyed three legions and ended the only serious attempt to make all of Germany between the Elbe and the Rhine a Roman province. It did not stop Roman punitive actions. A Roman army comprehensively dismantled Arminius' anti-Roman coalition a few years later by battle and diplomacy, and Arminius fled in disgrace to Boihaemvm, where he died. It is true that after Germanicvs' expedition, Rome never tried to reach the Elbe border again; apart from the conquest of the
agri Decvmates, Germany remained outside formal Roman jurisdiction.
The key, however, is the word 'formal'. Rome still, to all intents and purposes, ruled the territory beyond the Rhine-Danube border. Roman armies went on periodic rampages in Germany, annihilating political opponents and securing prisoners to be used as slaves or ritual execution. Roman gold backed compliant chiefs, whose tribes relied on Roman goods for their economy. Roman armies recruited soldiers from beyond the frontiers, who would return to their tribes with the power and status that Roman employment bestowed upon them. Life in "Germany" after Arminius revolved as much around the power of Rome as it did if he hadn't launched his rebellion. If he did anything to Roman-"German" (and speaking of the tribes in the region as "German" is a horribly anachronistic nationalist fantasy) relations, he kept them in the same state that they had been: Roman supremacy, with everybody else subject to the vagaries of their caprice.
Fritigern did no better. "Goth" here is again anachronistic; Fritigern was a chieftain of the Tervingi, not the "Goths". He led an invasion of Roman soil, beat up a single Roman army, and then saw his forces comprehensively ground down by the
still vast Roman military; he died before they could finally force the Tervingi into defeat. And defeated they were. Even the staunchest proponent of Gothic power, Peter Heather, concedes that the war that ended in 382 was a defeat for the Tervingi and Greuthungi; he claims it wasn't as bad a defeat as it might otherwise have been, but still a defeat. (And there are plenty of historians who argue his point by noting that there is no actual evidence in favor of his interpretation of a semi-favorable 'treaty of 382'.)
That you mentioned the "Goths" is a useful stepping stone to a further point, namely: the "Goths" were effectively a creation of the Roman military. They were military units raised from territory in the western Balkans, which had been partially settled by the two tribes mentioned above. They were not a proudly independent tribe that managed to retain their independence through military victory; they were an amalgam of various groups that the Romans considered to be militarily useful and which elected to adopt a unified identity
only when it became clear to them that
continuing to act as normal units of the regular Roman military was not as good a bargaining ploy as they'd hoped. They were born in the Roman Empire, and had been citizens their whole lives; they never knew anything outside the Empire, and like virtually all second-and-later generation immigrants in history they were perfectly loyal to their home country...just not, necessarily, the specific Emperor that happened to be running the show at the time. But in that they were no different from any of the "unambiguously" Roman players in the fourth- and fifth-century civil wars. Gothic identity, as constructed first by Alareiks and later by Evareiks and Thiudareiks, was only retrospectively applied to the earlier groups, like the invented Saga Kings of Sweden or the tales about aboriginal American history in the
Book of Mormon.
I posted on a similar subject in a thread a few months ago; it doesn't specifically apply to the Goths, but it explains the reason behind my comments and it works quite nicely by analogy:
And the remarkable thing was that the Empire died in spite of the fact that literally nobody wanted to destroy it. Instead, the various squabbling generals were each aiming for a better position within a functioning, powerful Rome. Many so-called "barbarians" - who were, in many cases, simply Roman-born officers in charge of Roman field armies, only some of whom had distant ancestors from over the border - were in the same boat. It was only after a century (380-476) of blatantly obvious evidence that the Empire was broken that they started to act less as Roman grandees and soldiers and more as the rulers of their own little pocket kingdoms.
Sussing out which of these men were barbarians and which were Romans is a bit of a joke. Historical convention, for example, labels the shadowy realm of Syagrivs and Aegidivs, centered on Noviodvnvm in Gaul, as the "Dominion of Soissons [sic]", a loyal Roman province that survived the fall of the West for some years. Its chief competitor was the realm of the "Franci", led by Childericvs and Chlodovechvs, which is described as a "barbarian" kingdom. Chlodovechvs eventually defeated Syagrivs, conquered his lands, and came to rule all Gaul, which eventually gained the name Francia after his kingdom, or France. Yet there was no meaningful difference between the two sides. Aegidivs and Childericvs were both born within the Empire, and led elements of the Roman military. Each obeyed and disobeyed various Emperors based on which way the political winds were blowing; they eventually both struck out on their own when it became clear that there was no real benefit to allying with whoever was flavor of the week in Italy. The only thing was, Chlodovechvs survived and had to create a founding mythos for his kingdom, so he drew on the "Franci"; Syagrivs didn't, so he was left as a Roman by default. The same situation obtained everywhere else. New identities were created wholesale, or elevated from obscurity to supersede a defunct Roman identity that lacked the same sort of cachet that it once had. Romanness did not cease to be afterwards - far from it. But it was a secondary layer of identity, not the primary.
When the end of the Empire came, it was not because some wooly, hairy, unlettered savages from the misty forests of the North came in and destroyed civilization. It was because the leader of the Italian imperial field army, Odovacar, once again launched a revolt against imperial authority looking for a bigger slice of the pie, and decided that he could get a better deal from the Emperor in the East than from the Emperor in the West. It was just one more sordid elite-management fiasco in a century-long line of them. And it was four decades until everybody decided that that had unequivocally been the end of Rome in the West, and that only happened because the Emperors of the East decided they wanted to have an excuse to conquer Italy so they'd better spread some stories about Italy being overrun by "barbarians".
Anyway, Fritigern wasn't some great Gothic-German national hero, he was a loser. And he certainly didn't reduce the Eastern Empire to effective vassalage or whatever. The Eastern Empire remained one of the most powerful countries in the world for centuries; it employed a mixture of direct financial and military means to suppress and/or deter its enemies, just like Rome always did. Direct financial means were cheaper and more effective; military means sated egos better and were better for propaganda while usually destabilizing things more than they fixed. The Empire and its subjects were still more prosperous than virtually any other European society until the Fourth Crusade.
Say what you like about Fatih Sultan Mehmet, and there are a lot of things to say about him, but he was an unambiguous winner. He killed Rome. If there is to be a reckoning with Tevinter, I would prefer that it be actually destroyed, and that is what he did.