There's still a bunch of fundamental changes to how the XML-based MSBuild files work, such as the addition of wildcards and recursive includes, so expect a wait for the rest of the ecosystem to catch-up.

Note that the move back to MSBuild from Project.json was prompted by pragmatism rather than ideology - I suspect the JSON-camp are biding their time before putting forward another proposal.

The Microsoft dev making the blog post is certainly explicitly saying so in the comments. :)