There was a limit to how much longer he would live. He certainly wouldn't be able to hold on until Skywind's Daomerge.
The Lonely King had said that if Ji Ning was extremely careful, he'd be able to live a thousand chaos cycles… but in truth, Ning knew that his Eternal Omega Sword Dao was actually the same thing as the prime essence of the Dao of the Sword, one of the prime essences of the Chaosverse. This Dao had a nurturing effect on his soul, which meant that the truesoul's crumbling speed would be fairly slow.
Different Daos would have different effects. For example, those who trained in the Dao of Water would have stronger protective abilities and invulnerable forms. In turn, the destructive Daos such as the Dao of the Sword were superior in offensive power.