I guess this highlights the difference between what the person in the street considers a successful politician (being a good leader, getting their policies enacted, leaving the country in a better position than you found it) and what a professional politician might consider being a successful politician (getting reelected, making your way up the greasy pole within your own party).
Boris Johnson wants to be Prime Minister and so becoming PM makes him successful - regardless on the impact on the country. His carefully constructed public image, and the lack of a "star" leading the Labour Party means that he can sail through on brand recognition alone.
When he gets his no-deal, no matter how dire the consequences, he'll find a way to blame others and the public will love him.