I think a protracted war of attrition is the more likely scenario. As long as Ukraine is given weapons they are willing to go toe to toe with Russia. I don’t see why things have to escalate to a WW3 scenario to be honest.
The Taliban held out against the US for 20 years and ended up leaving. Who is to say Ukraine couldn’t wage a low intensity insurgency to drain Russia dry…
All Ukraine has to do is not lose. Russia can’t sustain this tempo forever. Look at its economy and how much it spends on defense. They need a ceasefire more than Ukraine.