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.