If you're reading the novel, check the scene where the Don is giving his sermon to Bonasera. Sonny's not paying attention. The Don notes this, and fears for the family's future. He knows that he's not going to live forever, and it looks to him like Sonny isn't learning how to be a Don. On the other hand, the Don neither expects nor wants Michael to be his successor, at least not before Michael killed Sollozzo and McCluskey. He wants him to be a governor, a senator, a pezzanovante. But there's a hint of things to come. Check the scene where Paulie and Clemenza drive Kay back to her hotel after Connie's wedding. Clemenza says something about how the family has high hopes for Michael. Later, after Michael becomes the boss, the Don tells Hagen, "I never thought you a bad consigliere, I thought Santino was a bad Don."