They were just complacent, like the other empires that collapsed. I've been playing a political simulator game, and I realized that once people get powerful enough, they tend to collapse.
Sometimes they get decadent and compete via worthless luxuries.
Sometimes they feel they're invincible, and downplay or antagonize a threat until it's too late. Boredom, bloodlust, and overconfidence are fatal traits when combined together.
Sometimes they have an immense army and no more enemies, so instead of powering down their army, they use it to overexpand and conquer people without winning their hearts and minds.
Romans probably fell by a combination of these. I don't respect them too much, to be honest. Not a fan of decadent empires.
I think best were probably the Chinese, who have lived on through this day, and managed to repopulate the world without conquering anything. Followed by the British, who colonized much of the world and made English the world's lingua franca and took a couple of world wars to destabilize. Rashidun Caliphate was pretty awesome too, they defeated both the Byzantines and Persia in only 10 years after forming a single city-state with far less troop numbers and military history, then conquered an area larger than the Roman Empire within 40 years, and had a similarly permanent cultural impact.