A dusk of mutual ruin

During the Punic Wars, Rome was martial, institutionally mobilized, and possessed immense regenerative capacity. With 15% of its population killed in battle, it still united in shared hatred, carrying the life-and-death struggle to the end with perseverance and determination unimaginable to later generations, never suing for peace.
By the time of Heraclius, Rome had become bloated, demoralized, sharply divided, and torn apart by ethnic and religious issues. The Sasanian war was just the last straw. Soon it was completely vulnerable before the Arabs.
In history, we often see two such "old empires" dragging their gasping bodies onto the ring to fight. The result is mostly a dusk of mutual ruin, never a case of the East rising and the West falling. I like to call it "historical garbage time"—the Southern Song vs Jin, the Western Jin destroying Wu, all such cases. It's a contest of who is worse, who can drag on longer.
Isn't the world the same today? The Western world, long accustomed to comfort and privilege, seems to have forgotten that just a few decades ago, the United States could mobilize 15 million soldiers and build ships as fast as making dumplings? At that time there was an iron-like determination to confront—any bottleneck would be overcome; any sacrifice would be seen as glory; any retreat would be regarded as disgrace.