This question seems to be a different way of asking whether Russia will 'lose' its Caucasus republics

@JoshuaWilkes While that would be an obvious way for this to resolve yes, Russia and Japan have similar enough populations so that fertility or immigration policies can influence the resolution.

For example, if Japan were to be able to maintain a population growth rate of over 0.5% annually and Russia's population stagnates (both of which definitely within the realms of possibility), Japan would overtake Russia without any territorial changes needed.

How will a similar transition to USSR-> Russia be interpreted, where the former entity fragments into multiple smaller entities, but one inherits the former entity's positions in eg. UN and other political organisations?

@Toby96 I'll outline a few scenarios here to illustrate what I mean by "Russia no longer exists", they will mostly be based on what happens with foreign debt, but I do expect international recognition to align with the partition of the debt however.

1) The current Russian state continues, but a small region breaks away -> Russia still exists
(e.g. Chechnya breaks away from Russia, but nothing else changes)

2) The current Russian state breaks up, but one breakaway state takes on all external obligations, akin to what happened with the USSR-> Russia still exists
(e.g. All Federal Districts become independent, and one of them takes on foreign debts)

3) The current Russian state breaks up, but external obligations are divided/repudiated -> Russia no longer exists

Population will be calculated based on de facto control of land, so the city of Zaporizhzhia would not count towards the calculation right now for example.