Nathan remember Ancient Rome was defeated not by some massive other empire, it was worn down and failed to innovate. This is generally the lesson of history, empires are bled to death rather than receive one mortal blow from a compeating empire. In a business sense we have all witnessed the same many times over in the last 30 years, how many High Street shops and chains have closed because of the multitude of competitors that exist on the internet, those that have innovated and reduced costs not having to have expensive properties on High Streets in numerous towns with the associated staffing and running costs for each but rather a small warehouse (perhaps just a bedroom or garage) up to a utility warehouse where the costs are but a percentage of the High Street stores.
What is more is that any empire has its own limits, staff see opportunities and leave the 'empire' to set up their own small niche competing enterprise, management cliques argue and fight for their own control of the empire, these and a multitude of other internal factors start to weaken an empire, then external competitors constantly nibble away in different areas, and each small abbrassion on the surface of the empire has to be patched up costing resources and diverting effort away from growth and innovation. All of these amount to an empire having a life-cycle, some times a re-emergence or bounce may occur, but nothing is constant.
The size of the individual challengers does not matter, it is their combined size of their pressure and the amount of combined damage and costs that they cause Google that will likely topple Google from the No. 1 spot in the end.