Yup, the way you interpret the JSON or DICOM matters.
In DICOM, the interpretation of the data is coupled with a specific file format, when it doesn't need to be.
We want window leveling to work the same way across the board, but whether you transmit the width, center, and algorithm (linear, sigmoid, etc.) in a DCM file or in JSON or in carrier pigeon doesn't particularly matter so long as the receiver can interpret it.
Would it be better if all modality manufacturers started sending JSON? Probably not, however manufacturers produce non-compliant DICOM ALL THE TIME so there's still an issue there with complexity and lack of strong testing tools for the standard.