Why is a Ukrainian victory improbable?
Because the US is running out of money and patience and crazy Poland that sees Russia as if the Soviet Union was still in operation does not count.
Historically, when the US runs out of money or patience the end is near. See Vietnam and Afghanistan. That's why.
Also, Russia is not in the mood to give a chunk of itself to its enemy. The more casualties, the less desire to do it.
C, D, and L are Russia now and you can argue and quote all day long but to Russia protecting Russia and Russians is not negotiable. So the only question is how many people will die on both sides before that simple reality sinks in. How this works, read about Stalingrad and WW2.
Please don't insult me with "...but the referendum was not legal..." or any such crap.