The :migrate management operation creates a new subsystem and removes the legacy subsystem. It also adds an extension for the new subsystem, but it doesn't remove the legacy extension.
The legacy extension must of course stay in place in managed domain (the :migrate operation could be clever and check other profiles if they still have the subsystem, but I'd consider this behavior out of scope), but for standalone, I think the extension could and should be removed. Ideas?
Brian noted  that there might be technical obstacles in removing the extension, in which case I of course agree with rejecting.