Demographics compound an issue, they are not the root cause.
As has been said before, what did for the Imperium Romanum, both in 476 and 1453, was an unstable succession system that lead to civil war after civil war. Rome never quite recovered from the Crisis of the Third Century, and whilst Constantine did pull things back from the brink his successors ultimately poo poo’d all his good work.
Even in times of demographic contraction there were enough warm bodies to fill the legions. Indeed, given that the Imperial Army was larger than it had ever been in the 4th and 5th centuries, still mostly manned by Roman Citizens, I think the issue is overstated.
Given enough time these things correct themselves. Almost as if it’s Momma Nature’s way of preventing overpopulation…
It should be noted that in the later Dominate, the expansion of the military did present and issue, especially when combined with demographic contraction: there regularly weren't enough men to produce sufficient food in various regions, leading to increased dependence on imports from other regions. These issues ultimately contributed to a replacement of traditional armies by
foederati, which did address the problem, but created a new form of dependency...
Very much true that the Empire didn't really recover from the Crisis of the Third Century. Indeed, no universal empire in history has adequately recovered from its mid-imperial crisis. The attempted solution is invariably reminiscent of the Roman Dominate-- and can't properly resolve the fundamental issues.
Said issues, however,
ultimately stem from a geographic issue. The Empire reaches its limits. All Empires do. Even if you conquer the entire world, then the entire world is simply the scope encompassed by your limit. After that, expansion ceases. And how does the wealthy, glorious Principate become and remain so wealthy and glorious? Through a wealth pump mechanism. Expansion is enacted, new vassals or subjects pay taxes or tribute, and with this, the Imperial core finances its splendour... without taxing its citizens too much (or
at all, in some cases). The new subjects / vassals generally don't rebel, because they get military protection.
That protection has to be offered for
real. Its existence keeps the Periphery loyal and orderly. And as any & every Emperor in history has discovered, the wealth of the Empire attracts raiders. To stop raiders for real, the only way is to march across the border and pacify the region. Gues what? NEW VASSALS! Meanwhile, the old border is becoming "civilised". Its inhabitants gain citizenship. Meaning they, too, become part of the expanding Core region.
Eventually, it becomes impossible to conquer new peripheries. Logistical limits are reached. Rival empires are encountered. In the final instance, for a universal empire that conquers the whole planet, the sky is
literally the limit. (Space travel and terraforming are
hard.) So... no more Periphery. At last, the final frontier is hemmed in. It becomes part of the Core. Part of an Empire that is
all Core. Once that process is complete, the mid-imperial crisis hits.
The result is the Dominate, in which the citizenry becomes ever more taxed. No more wealth pump. The mechanism that fueled the Empire has broken down. The solutions of the Dominate extend its life, but do not restore its vitality. The Empire, at that stage, is literally on borrowed time.
(There are ways to escape such traps, obviously. But those methods require a degree of flexibility that universal empires invariably lack. After the empire collapses, smaller -- "more nimble" -- successor states will implement such methods and transition to different social and economic strategies that restore vitality.)
