I am guessing but requesting others to feel free to share your thoughts. Content fragments having composite or nested muktifields which again are composite are tricky in nature. May be the enhancement can support saving the data in desired format, but the ability to query using graphql may become an issue since graphql requires schema of the cf to be known before hand and introducing custom structure in between may break the design, and perhaps is the reason Adobe advises using nested content fragments to accomplish the same. I agree from an authoring standpoint this is a valid use case but Adobe may have to provide alternative means to meet both use cases.
This has been reported to the engineering under the internal reference SITES-15648. The product team will triage this request to verify feasibility based on the prioritization model. This post will be updated according to the Jira request status.