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JSON has a strong selling-point that it is compatible with JS in syntax.

It is very hard to make major extension to JSON and still be compatible with JS syntax. Minor changes are possible, like in JSON5.

Once it breaks JS compatibility, I don't think people will think it is JSON next any more.

Good point. If it's not JSON next but a complete new format than you will have to compete with JSON, all JSON next formats and all other alternative formats. Good luck.

PS: Answering myself - the leading data format might actually be the humble Comma-separated values (CSV) format! Love it really :-) Let's make it better and improve it - let's welcome csv,version: 1.1 -> https://csvalues.github.io

Isn't this what the author is trying to do? Mark seems to be a superset of JSON.

Looks like Mark is more in the tradition of YAML, that is, YAML is a superset of JSON too, e.g. it wants to be its own format (not just a humble extension). For example, my better JSON format flavor is called JSON v1.1 to make it clear its just humble JSON, but improved :-).