- After previous proxy wars that saw a country flooded with western weapons it was not unheard of for a new regime to turn around and sooner or later use all the weapons they got against western forces in turn (see Afghanistan for the most famous example). Governments do slowly learn the lessons of the past and are hesitant to give out too many weapons.
- Mobilization in Russia has seen hundreds of thousands flee the country and tens of thousands of young men have been killed or wounded. Even after the war ends and sanctions are eventually lifted, the demographic imbalances that Russia will face in the coming future will keep crippling its economy and army for decades. NATO doesn't care all that much about who rules the Donbas, but they do care about reducing the threat of Russia to its member states. Prolonging the war for another few years will see tens of thousands more mobilized Russians being fed into the meat grinder, making the future demographic problems that much worse.
- The Russian state falling apart and former provinces selling their nukes to the highest bidder would be a Bad Thing (tm), so we want to supply the Ukrainians with enough weapons so that they don't lose (and maybe win back the conquered territories) but not with enough weapons that they could conceivably make a big enough counter attack into Russia that it would fracture.