You are ignoring the primary objectives that Russia has already achieved:There is no solid guarantee that any "line of succession" would be followed if Putin were shot by his peers.
Ukraine has kept Russia out of the rest of its country and made some inroads into the four oblasts. The Russians are running out of men and material. Russia tried to take Kiev, kill Zelensky, and eliminate Ukraine. Now it is barely hanging onto the four oblasts.
1. Land bridge to Crimea; and
2. Ukraine will not join NATO.
Russia has no interest in occupying the rest of Ukraine.