Respectfully, I don't agree with your takes. First off, betrayal (even family betrayal) was not something that came to be only after Vito's passing. While Vito had no living brothers, there were plenty of people that were very very close to him that could have turned on him for various reasons, and thus was subject to a similar threat from Genco, for instance. Vito's attention to detail and the steps he took to avoid being careless forced him to "think as the people around him thought" and turn over every stone to see what lied underneath. Michael was perhaps too busy working on big deals rather than keeping his eye on the details of the inner workings of his family. Not keeping Fredo close enough was sloppy work on Mike's part (as a Don and brother) and could never have happened under his father's watchful eye.

And thanks for the welcome SC! It's nice to be surrounded be similarly obsessed people.