Anyone can install mediawiki. Not many people will be able to put the time and effort that the emacswiki community has to keep a site going.
About the only part of the article that I agree with is that it's time to start moving code off the wiki. Though this isn't a "deadline" type problem, it should simply be discouraged going forward and packages of interest should start migrating.
Indeed. I smell some personal bias/agenda here.
You can not kill off an existing , actively used system (however crufty it may be) without a fully functional alternate in place.