As much as I think it's annoying I think no trailing commas enforces good coding hygiene and basically forces you to use a real encoder rather than as hoc string manipulations.

You are talking about data transfer encoding. Configuration languages are a completely different beast.

This is such a bad take

Care to like, elaborate? Stopping people from doing brittle stuff like

    print("{")
    for elem in list:
        print('"key": "value",')
    print("}")

seems like immediate worth.

That's not a very big hurdle to overcome, though:

    print("{")
    contents = ""
    for elem in list:
      contents += '"key":  "value",'
    print(contents[:-1])
    print("}")