I tried to explain the story on de:Wikipedia Diskussion:Kurier as far as I understood it. The essence is: In German wikipedia, about 2009, the special characters list was introduced using code from the toolbar 2006. This worked fine for many editors. So they didn't use the 2010 Editor because its handling is much worse, specifically concerning the special characters. For this reason, in German wikipedia the 2006 Editor has been used by a very considerable number of long-time high-volume users up to now. You (meaning the Mediawiki developers) did not know about this wide-spread use in de.wp. You gleaned from "data" that this is only a "tiny fraction" of editors which might be correct in an international dimension, but definitely not for German wikipedia. Therefore, you decided to stop supporting the 2006 Editor but you did not foresee that this would mean that on de.wp the special character list would be disabled, too. This was the reason for the storm of indignation.
So there is obviously a problem of information and communication between Mediawiki developers and (in this case: German) community. I think that you are generally interested to know how the tools you are programming are used on a daily basis, but you did not have this important information. It would have been possible to detect this problem in advance since there are users (at least on de.wp) that foresaw it, but they didn't "come through."
Perhaps a kind of "ambassador" is needed: someone who ensures that you get the information you need from the communities and who ensures, on the other hand, that users without technical knowledge understand what a technical change means for them. Maybe it's a kind of language problem: not only English vs. German, but more so developers vs. (non-tech) users.