To be fair, I have never understood this obsession with Ukrainian numbers as some kind of proof that Ukraine is doomed. There are people that have sent their families abroad and returned to Ukraine to fight Russia. There are certainly also ethnic Russians that do not want to help Ukraine in this war, but on other side they do not want to join Russia either, as they would be sent to die for sure. Many reasons for many people what they do and why.

During WW1 my home country lost 50% of population, as many people escaped frontline - most of them returned and rebuilt everything.

I am very sure that Ukrainians after war will rebuild Ukraine - probably that will be another European center competing with Germany - unlike Russians, that are abroad now as they won't rebuild Russia - as Russia very surely will collapse. What would be built in the place of Russia is rather hard to tell right now, but I do not think that many Russians will want to return to that where they would be out of place in those new countries, just like they are out of place in modern Russia.