There was nothing wrong with csproj, apart from the MSBuild verbosity baggage. JSON's a terrible format for anything like this due to lack of comments and visual noise. (The mandatory quotes all over the place makes it hard to read.)
But I can't imagine it is hard to convert between them. Most .csproj's are basically a list of references and a <Compile><Include>*.cs.
While some people get a bit cargo cult about things there is a rough consensus that XML is good for some things and terrible for others whilst JSON is the exact reverse ;-)
I personally rather like YAML for anything intended for humans to read and edit.
If XML didn't have those idiotic verbose closing tags, this probably wouldn't even be a discussion. Or if JSON had comments and didn't require quoting all keys.
What's funnier is seeing the HTTP/JSON folks re-invent SOAP. But maybe this time it'll actually be Simple.