“Military cuts
Though militarily superior and marching on advanced road systems, the highly-trained Roman legions were strained fighting conflicts from the Rhine River to the Sassanid Persian Empire. Roman borders were over-extended and the military defending them was cut back to dangerously low ranks.”
Maybe, to reduce the effect of other factors like debt and high taxes, Rome should have focused more on itself, instead of policing and Romanizing the world with its decadent culture. Maybe its “exceptionalism” was the virtuous part of its culture (its art and law and engineering). But it was overcome by hubris of empire and war, the distraction of it all, and the sheer impossibility of its scale.
However, from the ruins, Christianity could fully emerge in its wake. That even kept Islam out of Europe. Until today...
Maybe if the sons of Roman leaders were the ones fighting the wars, they’d have been less likely to send them hither & yon? Alas, like the Romney and Sununu families, they were quite happy to let others fight those wars.