Interesting idea! The regex would need a little extra care to interoperate properly with prefix_chars, but the approach doesn't seem crazy. I'd probably call the constructor option something like "args_default_to_positional" (the current behavior is essentially that args default to optional arguments if they look like optionals).

I'd be happy to review a patch along these lines. It would probably be good if Anders Kaseorg could also review it to make sure it fully solves his problem.