I worked on the MS Word core team for a little over three years from 2010-2014, and de-facto owned a significant part of implementing ODF / OOXML Strict support.
The binary format was a liability for Microsoft to begin with, because of decades of cruft lining up with actual memory alignment. During my tenure there I ran into code my GM had written as an intern and was still intact -- he had 20+ years of tenure (mostly on Word) when I joined the team.
The translation of the file format to XML involved a significant amount of performance degradation if you weren't careful. Hundreds of millions of people use the app monthly, and MS still tries to maintain backwards compatibility. Given that open APIs were a relatively late development for the app, I really don't think in the current reality of what's expected by boards of directors for the companies they oversee that _anyone_ would take years to:
a) define a spec that maintained that backwards compatibility
b) reach whatever nebulous simplicity metric today's HN article wants
c) not get whoever greenlit the project fired for taking that many engineering hours for a and b