The PRC had won the civil war and controlled the vast majority of the land/population of China. Using common sense, PRC was the successor.

Common sense also says Russia is the successor to the USSR as it contains the majority of the land, people, capital, central historical institutions that made up USSR.

Finally, the UN already selected for Russia 30 years ago, its a done deal. Theres no going back.

Trying to claim ukraine is the successor to USSR is the same kind of logic with which trump tried to steal the elections. Lets stop trying to warp and bend the rules/laws to however we like it. If we make this precedent, one day it will bite us in the ass.