Didn't Luffy lose his conviction after his brother died? In fact he almost gave up on his journey if I recall.

Luffy doesn't have as much character progression as someone like Sanji, who constantly has to reconcile his past with the person he is now or Usopp facing more and more of his fears and overcoming them. The way Luffy's character is written means he is almost unwavering in his ideals but Oda does flesh him out as much as he can in lieu of it. Luffy easily has the most extensive backstory out of any of the Strawhats and a complicated a family dymanic that Oda often likes to play against. I'd say Oda writes him well enough within the confines of his established character.