I still think this is the most likely scenario. Unless the person who eventually replaces Putin is not another strongman but someone with a real commitment to democracy, someone who wants Russia to join the EU, and who's able to convince Russians living in both Russia and the Ukraine that the present Russian/Ukraine border should stay where it is forever. Which is very, very unlikely, IMO. Russians still remember that the eastern Ukraine was part of Russia until Khrushchev made it part of the Ukraine (and they don't think it's relevant where the Ukrainian/Russian border was in previous centuries).