The question as to why empires never last forever is an interesting topic: there have been many along the timeline of history - the Roman Empire, the Byzantine Empire, the list is never-ending. But whilst these may seem mighty, they have one thing in common. They have all crumbled down to just a memory, over hundreds of years of their existence. All of this begs the question of why?
Empires are grand and majestic on the outside, yet straw-thin on the inside, causing their collapse eventually. What usually happens is that another set of people come up, with fresher and newer ideas than the current empire, and challenge that current empire, historically in the form of a battle. An example is the Roman Empire, when the Romans got overthrown by successive attacks from people known as barbarians, but this name was given to them mainly because they were seen, by the Romans especially, to be barbaric towards Roman tradition and rule. But, yet again, another strong empire defeated. This all goes to show that, although the empires might be rich or generous, there will always be haters.
Another example is of the British Empire, where countries were being very badly treated under British rule, but they did not have enough poewr at first to revolt. However, after years of struggle, the British were finally overthrown, - by Alexander Hamilton, Mahatma Gandhi, and many more - till all they held was their own island of Britain.
In conclusion, empires never last, as there is always a growing unhappiness with the rulers, or laws, or something of an entirely different matter. Examples I have described include Rome and barbarians, Britain and the peace of India. Once again, this is why empires never last.