What gives any arbitrary country a mandate to exist though? We recognize that plenty of other social institutions are transient. Nobody demanded a massive international intervention to continue the existence of the Whig Party or the Studebaker Corporation. Why are countries unique and special? Also, this seems like a very modern thing: nobody is demanding we bring back Tanganyika or unwind German unification.
I get the desire to preserve the Ukranian culture and community. But you don't need a sovereign nation for that: compare the re-establishment of the Welsh language and culture, for example.
Would the population have been better off-- at least in the "not exploded" sense-- by backing down with a quick surrender in exchange for some "we'll formally tolerate your cultural differences" legal concessions? I'm sure at this point, it's impractical to negotiate to that, because there's too much bloodshed and burnt bridges on both sides, but it seemed like it was never even on the table: the Western world decided an independent Ukraine had to exist even if everyone involved knew it was going to be a very painful and expensive endeavour to keep it.
One of the key differences to other instances is that Ukrainian culture was not really tolerated in the USSR, and definitely not in the imperialistic cute of Russia today. Rather, Russia learned from the demise of the USSR that it's better to remove cultural and societal differences within its nation. And it's going the same route as other empires have in the past and are doing today. Enforcing the use of its language, forcibly educating the youth in Russian culture and schools, often by literally abducting them from their Ukrainian homes and relocating them to Russia into Russian families. Kidnapping a people's children is literally one of the five acts that make a genocide, according to the UN genocide convention from 1948.
There are many examples to show that putting peace above all else is dangerous. Using your example, the Welsh were violently suppressed and exploited for centuries by the English, following a similar scheme. The Welsh language was forbidden to teach, Welsh traditions were replaced by English, and power of autonomy was transferred to the English. Nowadays less than 20% of the Welsh can speak their language, and that is after the "resurgence" and it being allowed to be taught again.
Other examples are Native Americans, the Armenian genocide, Czech Sudetenland, Poland, oh, and remember what China is doing in Xinjiang? All of them posed no or little violent resistance in exchange for promises of peace and cultural autonomy. It rarely takes more than a few months before contracts with powerless people are broken.
Nations are just figments of imagination, but are an expression of communal will of a number of people. Modern ( as in last two centuries) concepts of human rights revolve around the freedom of people to choose how to live their lives. We usually consider it admirable if people are allowed to freely live their chosen cultures and tradition. We also consider it proper that people are allowed to choose what kind of society they want to live in/migrate to. We also grant asylum to those forcibly prevented from living their way of life and being persecuted in their home nation.
Nations are a construct allowing specific sets of societal rules to be applied to a large populace. An internationally recognized nation is also considered integrally protected, even if one nation might not agree with the internal societal rules of another country. These global base rules are very bare bones, but they are one of the big reasons we all get to live in the era dubbed "long peace". Yes, there's still wars and ugly conflicts, but at least there has been no major total war involving superpowers. Even the Ukraine war is luckily still limited in scope. Were Russia to unleash their full military might, it quickly would devolve into a humanitarian disaster not seen since the world war.
Why does the arbitrary nature of countries favor the invader though? What about the argument that Russia is an arbitrary country that doesn't need to violently expand into neighbouring countries?
I'm going to assume that you're arguing in good faith and aren't just a troll.
I myself am very anti-war, but I tend to take a more practical standpoint, because in order to avoid war, both sides must work at peace, not just one.
There's a few problems with your line of thinking.
For one, you suggest that Russia formally agreeing to tolerate Ukrainian culture would be enough, but suposing Ukraine did back down on that condition, how could they possibly trust Russia to stick to their word once any bargaining power they had is gone, especially since Russia has previously recognised Ukraine as a sovereign nation and had no problem ignoring their own word on that.
That brings me to problem two, or modern day notion of nations is relatively recent, but it does seem to work. Before the world wars, there was always borders changing and this was seen as normal. But since at least world war II this has changed, nowadays nations choose to recognise other nations right to exist with a given set of borders, this is a fragile system and if we simply allow countries to arbitrarily go against it without any repercussions, then why would any other country abide by it?
Also if Russia and Ukraine could agree to have Ukraine become part of Russia, I, as someone from neither country, would have nothing particularly against it, as long as it was entirely peaceful diplomacy, Russia removed that option the moment they started gathering troops near the border, because at minimum, that's a threat.
I am no expert on the history involved here by any means, so if I have made any mistake, feel free to correct me, I'm simply thinking through the logic.
Edit: I'd like to add that I hope you die today and that your pets eat your face and die from all the poison in your body, you shit eating God damn worm.