This is, by far, the most disgusting and disrespectful action undertaken by the Foundation that I have ever witnessed. The 2012 mass desysopping of volunteer administrators on the WMF wiki and the past threats of desysopping users re: VisualEditor and MediaViewer do not even come close to this.
It is clear to me that the Foundation has agreed on this sneaky change behind closed doors while some of the most outspoken Wikimedia volunteers were (and still are) gathered in London. This is not the first time that we're seeing this happpen, and it is clear to me that the Foundation has lost all remaining moral authority to talk about transparency and involving volunteers in the decision-making process.
Erik has forced his employees, including a so-called community advocacy liaison, to use this opportunity to actively fight the volunteer community of the German Wikipedia. He himself has engaged in a wheel war over this, and continues to shove MediaViewer down the German Wikipedia's community throat.
I'm not sure what was the purpose of this change, but if its aim was to escalate the already tense situation between the WMF and its volunteer communities regarding MediaViewer, protecting the MediaWiki:Common.js page so that no one can edit it was the perfect choice.
This action will cause a huge shitstorm, and Erik deserves every bit of shit and mud that will be thrown his way.
You can force anything you like on your employees, but you cannot force the volunteer community to do what you want, not in a manner like this.
Remember that in the end, the community can exist without the WMF, but there is no WMF without the community.