I think you're taking all of the decisions that led up to the success of the iPhone for granted. For instance, if this path is such a no-brainer, why doesn't Microsoft install a walled garden of their own? It's not due to the beneficence of their hearts.

Something that is a surefire success in one platform is not necessarily a surefire success in another. Which is why saying in hindsight "this is monopolistic behavior!" is such a silly thing to do.