Adding to what Skizzerz and Siebrand said: there are "only" 306 extensions in SVN of which we didn't know the status (including usefulness) and back then I contacted all their authors: https://www.mediawiki.org/wiki/Git/Conversion/Extensions_still_in_svn#Unsorted_items If you see so many cases of extensions which should be migrated because people want to use them but are having problems, you could make a new list. http://wikiapiary.com can also help you select further SVN extensions of relevance. It's also useful if you tell users to file bugs for each extension they really need to get migrated. When we have more information on the problem, we can see if a change in process is needed (e.g. a proactive search for maintainers for some extensions).
Nemo
P.s./premise 1: unlike Siebrand, I think most users will *not* get themselves heard. That's why with WikiTeam I've been spidering the web for years, to acquire all the information possible. It only took me 300 $ to get the last list of wikis: https://wikiapiary.com/wiki/Category:Oct_2014_Import. Now go use the data! P.s./premise 2: it's a bit disturbing to hear "volunteers X _should_ be interested in Y". The MediaWiki Cooperation is open for new members and none of the above requires superpowers nor a bolla papale; if you think an activity should be done under that group, join the group and to the activity.