You’re failing to separate “Ukraine” the concept of a nation from “Ukraine” the government from “Ukraine” the group of people. In material terms, if life as part of the Russian Federation is comparable to life as an independent Ukraine (and Crimea was doing alright when there wasn’t much fighting), what does a war accomplish? Especially a war that the Ukrainian military was always going to lose. The end result is, as Yogthos said, incredible human suffering without even accomplishing the worthless task of keeping your preferred flag flying.

Now, I don’t actually agree that Russia wants to annex all of Ukraine. I think they want to break the back of the military and annex as much of Ukraine as votes to join them, but handling the colonial occupation of a country that wants to secede (like Ukraine had been doing with Donbas) is not in its interests. That said, even if they just wanted to take over the whole of Ukraine, a negotiated peace that wins concessions for human welfare would, in every respect, be a superior result to a losing war unless you’re a dog of the west and see damage to Russia as worth throwing generations into a meat grinder.