The specification indicates that .gender is "semantically vague" property in FHIR today. I think there has been significant efforts to make this clearly Administrative Sex. I would far prefer that any perceived vagueness be clarified in FHIR rather than adding an extension for this precise purpose. 

considering .gender to be needed to be maintained for backwards compatibility at this stage in the specification is not an appropriate pathway.

This said, I do recognize that the new extension carries more information such as period that are not on .gender today. Thus I do expect a role for extensions. 

Seems that .gender should not just be kept around for backward compatibility, but have a defined and required relationship to gender identity.