[dmm]

You're assuming that the populace is happy with their current government. Some of them might welcome the chance to throw off their cruel, incompetent, failing dictators who make them live in domed colonies under harsh conditions. They might be excited to join up with a large, thriving, well-run empire with numerous rich planets of just the right type for them to settle down and make a good life for themselves and their children/buds/seedlings/clones.