On the other hand a decisive win for Ukraine followed by western-integration success story there - would directly contradict Putin's core internal justification for keeping his power (west is lying ,democracy is bullshit, ruskie people are different and if we try democracy it would just be 90s again). If Ukrainians have democracy and are more successful than Russians (and Russians would know - they have families and travel) - this whole system (not only Putin - but all his FSB friends, oligarchs, orthodox church, state media etc) - break down.
Remember that they are constantly telling their people that Ukraine doesn't exist, Ukrainians are just Russians brainwashed into believing "western lies". If these western lies WORK - it's the end of putin & company.
This is the most likely road to a democratic, constructive Russia. Such Russia would be a massive boon for EU and NATO, both Russia and its neighbors wouldn't need to waste billions of dollars on weapons. And of course it would be a massive improvement for Russians. Actual rule of law. Local self-government with reinvestment of oil money into infrastructure & education. The possibilities are endless. But it would require Russia to go through the imperial cycle to its conclusion - decisively losing a colonial war, realizing we're the bad guys, dealing with the history, etc. You need the defeat for that.
If we keep Russia alive we're making this very hard. Much more likely it will become a big North Korea.