Taking 40M people with the same demographic problems gives you a bigger problem - not a solution...
https://www.populationpyramid.net/russian-federation/2020/
https://www.populationpyramid.net/ukraine/2020/
In 10 years demographic in both countries will be as bad as in Japan.
For some reason people from outside of ex-USSR usually suppose there is at least something rational about this war. But USSR does not exist for 30 years and there are no massive political machine. It's just a autocracy / kleptocracy where all decisions taken by a very small groups of incompetent people whos only goal is to preserve their power.
Neither there is any real ideology in Russia. Putin is a much closer to The Dictator movie than to Stalin or Hitler. He care of his giant palaces and yachts far more than about gaining 42M extra subjects to rule. This whole war is just a presidental PR campaign 2024 gone wrong.
PS: Yeah, that's it. It's not some great powerplay to control the world. Not a war with NATO or US. Not even a war over reasources or territory. It's just aging kleptocracy that stumbled and now is killing 100,000s in pointless war.
The deal is: neither Putin nor population of Russia give a damn about this territory control. If Ukraine will free Donetsk and Luhansk tomorrow and Crimea in a year no one in Russia will care about it. Except of course people living on occupied territories, but FSB will shut them up.
On 30 September 2022 they announced annexiation of Kherson. Claimed it's will be part of Russia forever. Just a bit more than month later on 9th November 2022 it stopped being part of Russia and no one cared about it since. And too talkative members of occupation government just died under strage circumstances.
Please don’t invade any countries even if they’re smaller and inconvenient to your stories.
Not to mention, it’s a large landmass that offers a huge buffer between the NATO countries and Russia.
And it’s got another 40 or 50 million people for Russia, which has a population problem.
And it’s got iron & Steel exports…
Taiwan has TSMC. And I suspect the US has a bunch of fueled up C-17’s ready to take all the important infrastructure to the US and plans in place to effectively destroy the rest at a moments notice.