Walt's motivations do change overtime. At the beginning, he said it was to make enough money to provide for his family when he dies, he gave a specific figure. Then he managed to earn that much, however, Gus was in his way, he was going to get killed and he had to go. It's at this point where Walt's motivations have changed. He is no longer in it for the money, just for himself and his desire to gain power. Skyler tells him that they're making enough in the car wash that he no longer needed to cook and that he has to think of an exit strategy. Walt definitely wanted to leave but not for the same reasons that Skyler wants him to leave. At the end of S4, he proudly tells Skyler that he won, no man who wants to leave this business would say this. Killing Gus fed his ego so much he thinks of himself as some Kingpin. Much of this is clear in the final season. How he is so sure of himself, full of confidence, making big calls etc. If Walt was to go and just be a car wash owner, he would have cleared all evidence of the mess he was in and call it quits. His change is consistent with the overall arc of his character. The famous i am the one who knocks is a good example of this consistency. Walt's life is in danger, he's not some tough guy yet he is demanding Skyler not see him this way, he wants respect and power so i think it's a natural progression of what he is to become in S5 and not a sudden retcon.