which will have a conventional ending with one side as the victor
I don’t think anyone thinks that. This isn’t likely to “end” in our lifetimes now. Ukraine might be able to push Russia back to its borders, but it, and many other countries, will be in a state of readiness for a return to warfare for decades now. It is going to be a drain on all those countries, and other countries, including the UK, for a long long time. Even if there is a peace treaty, no one trusts Russia to stick to it. The current fighting can be ended in what you might describe as a “conventional” way, with paper agreements on borders (old or new) and military deployment (withdrawal by either/both sides and new border forces), but that isn’t the end of this… we are stuck with a weak and unstable Russia who could strike out at any neighbour at any time. And, yes, a nuclear threat is real, but that doesn’t end with a peace treaty either. It remains while Russia is in the state it is. It will outlast any ground war in Ukraine. Russia remain a threat no matter what the Ukrainians or any other country does next. They would be a threat if allowed to take all of Ukraine. They will be threat if pushed back to 2014 borders. They’d be a threat if Ukraine was carved in two with a new border and a much small country. The threat from Russia isn’t going to be ended any time soon. It is the new reality we live with. And that is all Putin’s choice.