A powerful nation couldn't rely on the abilities of only a few people.
Ling Han, the Rain Emperor, and the others all strove for great things. This was especially the case for Ling Han, who also had to find the Roc Palace and the ancestral grounds of the five sects. How could he remain on the Joint Peace Planet forever?
Thus, they had to work hard to groom successors. Since they had split the sky and brought the small world into the Immortal Realm, they naturally had to give her a brighter future.
Everyone improved in leaps and bounds with Ling Han's selfless help.