On the other hand, the Eastern Roman Empire lived on with varying degrees of success under Emperors such as Justinian and Basil. It lost its African and Middle Eastern provinces to Islam, but it maintained its strength in Asia Minor until it was further weakened by the Crusades and eventually fell to the Turks, 1000 years after the fall of the Western Empire.
The tax rate in the Eastern Empire was 3%, which most of the populous willingly paid for protection from invasion from the east.
The tax rate in the Roman Empire was 10%. In the 3rd Century, with the cost of administering and securing the empire rising, the government took to collecting the tax every other year instead of annually, then every third year, and finally every fourth year before it completely collapsed.
No matter how powerful your army, you cannot go into remote regions and attempt to suddenly take 40% of the populations Gross Domestic Product. Particularly when the population perceives little benefit from the empire. And your army is composed of fewer and fewer Romans and increasing numbers of subjugated people. None of this factors in the every present theft, extortion, and corruption of the tax collectors themselves. Revolt is inevitable.
On the other hand, the Eastern Empire survived until 1453 AD.