Putin is 71 and doesn’t look particularly healthy. It’s not clear that he has any strong successor, nor that he would leave such a successor alive if there was one. His government is strained to the breaking point and couldn’t fight off an internal military attack by one of his lieutenants, to the point where they almost reached Moscow in a few hours. When he dies, it seems likely that the country will fall to pieces as different factions try to gain power. Because of the way Putin has suppressed the opposition, Navalny is one of the only people who has any popular legitimacy in that environment. I still think it’s very unlikely he makes it, but prisoner to leader isn’t unheard of.
