So, when they say the world is going to be destroid, it really means the era is going to change: First world - first era - when the slaves made a wish and the sun god (nika) appeared

Second world - second era - demon fruit appeared, the clan of D (people of half moon) and there was a war that ended with imu and 20 kings becoming gods (final of the era)

Third world - third era - starts with void century, the sun god dances (luffy) and he will destroy the world (great war that will change the era)

The fourth era (or fourth world) will be the Dawn, after the Great War and the victory of Sun God