I think that JSON is a bad choice here.

It is obvious that CLI commands should produce machine-readable output because they are often used in scripts, and accept machine-readable input as well. Using arbitrary text output was a mistake because it is difficult to parse, especially when spaces and non-ASCII characters are present.

A good choice would be a format that is easily parsed by programs but still readable by the user. JSON is a bad choice here because it is hard to read.

In my opinion, something formatted with pipes, quotes and spaces would be better:

    eth0:
      ip: 127.15.34.23
      flags: BROADCAST|UNICAST
      mtu: 1500
      name: """"Gigabit" by Network Interfaces Inc."""

Note that the format I have proposed here is machine-readable, somewhat human-readable and somewhat parseable by line-oriented tools like grep. Therefore there might be no need for switches to choose output format. It is also relatively easy to produce without any libraries.

Regarding idea to output data in /proc or /sys in JSON format, I think this is wrong as well. This would mean that reading data about multiple processes would require lot of formatting and parsing JSON. Instead or parsing /proc and /sys directly, applications should use libraries distributed with kernel, and reading the data directly should be discouraged. Because currently /proc and /sys are just a kind of undocumented API.

Also, I wanted to note that I dislike jq utility. Instead of using JSONPath it uses some proprietary query format that I constantly fail to remember.