PNG coupled with a JSON can do almost anything that DICOM can do. DICOM can also do video and is starting to dabble in waveform data, but MP4 and SVG exist.
JSON is a data format, it isn't in itself a standard for any content in particular, particularly medical imaging content. Think schema, not encoding (and in fact there is a JSON encoding for DICOM).
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.
This is much less of a problem of shiny-object-syndrome not providing a "better" way of solving the problem(s) and much more one of market inertia, IMO. Many people overlook this.