I'd like to challenge you on that premise, actually.
You yourself have just said how difficult it would be to create surgically precise borders in places where there has been a lot of ethnic intermixing on territory. As you also said, the Balkans have a history. As soon as borders start switching, the peoples in the Balkans have a history of things spinning out of control and leading to people slaughtering each other.
Would that not also open up the potential of pre-World War II principles of might makes right and play right into the hands of the Kremlin when it comes to their case on whether it's Crimea, Donbass as you referenced, Transnistria, South Ossetia, or Abkhazia, a very different world order?
While that may seem to be small and insignificant, it has two extreme dangers. First, it could be a domino effect and things could go seriously wrong, and of course, the Kremlin loves it when chaos occurs. They've proven themselves, in many places in the world, to be very adept at working in those circumstances. Second, it undermines that principle of the unviolability of orders.