I guess Western part may be left alone, just heavily mauled. In the rest of the country, the insurgency was originally pro-Russian. Why should anyone miss Ukraine now? Do you realize that about 40,000 within the Russian forces are Ukrainians who hate the regime?
“I guess Western part may be left alone, just heavily mauled.”
No, sorry, as you said: “Nobody is going to let a hostile regime to stay on the Russian border anymore.”
So if Russia leaves the Western Ukraine, they still have a hostile regime on their border. And if Russia takes ALL of Ukraine, then they just have a hostile population INSIDE their borders.
The only options for Russia to avoid endless guerilla war and terrorism is to either genocide all the Ukrainians, or to just go home.
The rest of your post trying to deny there would be resistance is just pure fantasy. If that were true, then the Russians would have been welcomed when they drove up to Kiev, but everyone knows how that worked out.