Military reforms did occur (a defeat is always the best way of doing that). And I'm pretty sure that a generation of peace between Adrianople and the next major attack qualifies as "time to regroup". Victory at Adrianople means that the Goths are better assimilated in the Balkans, and maybe less of a necessity to transfer troops from less threatened theaters. Whoop-dee-doo. They would still have got hit by the avalanche of barbarian tribes in the first few decades of the fifth century, and the fact that the East is sitting relatively pretty has no effect on the ability of the West to resist those invasions. Morale had nothing to do with it; this isn't the twentieth century, where Vietnam reverberates with the American public for decades. It's the fourth and fifth centuries, where most of the inhabitants of your empire don't hear anything about military victories or losses. Tactically, the Romans still had a massive advantage anyway, so long as they didn't do anything monumentally stupid. Radagaisus, with the largest invasion in the history of the Empire, got thrashed by Stilicho in northern Italy and chased out. So morale issues don't make any sense. It's the strategic problems of having to deal with the usual barbarians they know and love, except those barbarians have had the benefit of technological progress in the last few centuries, thus becoming very populous and better armed. Then the Huns start moving and kick off the avalanche.