Is there anything that can be done by editing TemplateData JSON as text that cannot be done by editing it in the TemplateData editing form?
The only such thing I can think of is introducing invalid JSON, but that's a bad thing.
If there is nothing else that can be achieved by raw JSON editing then it be deprecated, and editing TemplateData should eventually be possible only through the form. This should make it more structured and less prone to errors.
This should probably go along with something like T56140: Move TemplateData to its own JSON-content namespace and associate with Template-namespace, or to its own TemplateData content model and revision slot.