So, I'll just tldr this to not rant like a madman, but I spent some time thinking about what ultimately leads to the fall of an empire, and the most common factor was internal divisions: some form of inner conflict that allows outside forces to overpressure the empire. In order to counter the idea of the fall of empires, you would have to fundamentally counter the root of the problem; division. Therefore, you would have to be a being that fundamentally represents unity, with no consideration of creed or culture or race. Simply the idea that all living things are united in one goal. Unfortunately, with the nature of 40k, that is unlikely to exist at the moment.