Attempts to alter the size of a shared memory segment are met with a variety of different, nuanced behaviors on systems we want to support.  I agree that it would be valuable to be able to effectively realloc a shared memory segment, which thankfully the user can do with the current implementation although they become responsible for adjusting for platform-specific behaviors.  The design of the API in multiprocessing.shared_memory strives to be as feature-rich as possible while providing consistent behavior across platforms that can be reasonably supported; it also leaves the door open (so to speak) for users to exploit additional platform-specific capabilities of shared memory segments.

Knowing beforehand whether to create a segment or attach to an existing one is an important feature for a variety of use cases.  I believe this is discussed at some length in issue35813.  If what is discussed there does not help (it did get kind of long sometimes), please say so and we can talk through it more.