Okay, there are a bunch of reasons against text-based formats. But what are the reasons to use this? This looks like another homebrew mostly-binary format, that makes one's eyes bleed when it has to be debugged.
A missing use case in both JSON and Muon: tabular data.
JSON was made for javascript, rather than a useful or convenient data exchange format.
It doesn't even show an example of a json file and its equivalent in muon.