With imperialism there are two paths: 1) You go whole hog and completely annihilate the system your taking over down to the last man. This doesnt work out because to do that you destroy everything no spoils of war. Additionally everyone else sees you do this and immidately forms alliances against you potentially to the point they rival your own power because you became an existential threat. 2) You dont go whole hog you absorb the competing system into your own after a short military conflict and concession. Well you see the problem with this? You end up integrating this alien culture into your own. Now they have power within your system, voting rights, some level of autonomy, there's cultural population exchange, people start mixing because of proximity and necessity now to integrate these two systems etc etc... Eventually the empire does this enough to the point that there is no core dominant culture or value system and is rife with foreign entanglements and it destroys itself.