I don't understand this. ContentMixin comes directly from PulpCliCommand, but the succeeded method is defined in PollingCommand. I think it explains why I didn't pick it up in the refactor, InstallCommand is the eventual PollingCommand subclass and it doesn't override anything that had changed.

The abstraction here is wrong; it's not internally consistent. ContentMixin won't run unless it's paired with another superclass that defines the succeeded method. It'll work assuming you get everything right, but since it's not technically an overridden method we lose any IDE inspection capabilities on it.