For a quick glance, I didn't see the corner case where the old-style `%` interpolation (may) shine over PEP750: building up a JSON document with a dictionary:

>>> hello_world = {"hello":"HELL" ,"world":"O'WORLD"}

>>> json_template='{"hello":"%(hello)s","world":"%(world)s"}'

>>> print(json_template % hello_world)

{"hello":"HELL","world":"O'WORLD"}