Russia cannot be fully “undone”.

Ukraine’s goal cannot be, and is not, to “defeat” Russia.

Ukraine’s goal, correctly, is to increase the cost of this war sufficiently that Putin’s underlings and oligarchs will revolt and overthrow him and end this incredibly wasteful hubristic endeavor by Putin.

Hold my beer.

Why not? People thought so about Soviet Union too. Just keep pressing until it dissolves, then reform away what's left until shards of the former empire is inhabited by people identifying as something else and no one speaks Russian at home, and sees Russia as a former occupying power.