I agree that stagnation and lack of progress is a real danger for a unified world, but there would be challenges open to such a world that could help mitigate this. One of these is an open-ended exploration, and ultimately colonisation, of the universe around us (quite independently of whether or not there is a perceived threat from aliens!).

Indeed, I have argued elsewhere:

http://www.homepages.ucl.ac.uk/~ucfbiac/Space%20Interests_files/jbis_culture.pdf

that a positive feed-back loop may exist between space exploration and world government: a future world government my require space exploration to avoid stagnation, but equally only a world government may be able to afford to engage in space exploration on a sufficient scale to make a difference. Moreover, the 'cosmic perspective' on human affairs that will inevitably follow from a large-scale programme of space exploration may help make the cultural and psychological case for a unified humanity (and thus for a world government.....).

Incidentally, although the word 'singleton' used here appears to be a new coinage, the idea of world government has a long history in Western political thought. See, e.g., Derek Heater, World Citizenship and Government (Macmillan Press, 1996).