Major and Cameron weren't always expected to go the distances they did either.
Smith and Miliband never became PMs now did they.
TL,DR: Don't count out BoJo yet.
Both of your examples deal with election results after not having had periods as turbulent and dissatisfactory as Boris is having, and in Cameron's case there was no internal Tory challenge at all. (I assume you meant to say Kinnock rather than Smith). This is about the likelihood of Boris actually making that election. For our country's sake I hope that election is quite soon.