I don't think they're proposing changes to Mediawiki, they're proposing changes to the look of Wikipedia. While they may be synonomous, one is the software, one is the organization that uses the software.
Wikipedia could change the template for their version of the site, without affecting MW at all.
He's proposing to change it so it works like Etherpad. That would have to replace every single byte in storage and all the previous versions. Think about it.
Wikipedia could change the template for their version of the site, without affecting MW at all.