- This is a personal essay.
With regard to MZMcBride's statement on 'retirement', "Short of real-life death, it's incredibly rare that someone actually leaves this place": that may be changing. Who knows what the reason is for the macro trend; we'd probably have to do surveys to find out, and even then accurate results would depend on people's self-insight. So I will focus on my own experience.
I've retired from English Wikipedia (enwiki), or committed suicide by sysop, innumerable times and come back. Nobody was able to chase me away, even by summarily deleting, pursuant to G5, slews of articles I created and reverting almost everything I did, whenever another sockpuppet was unmasked. I was undeterred, because a lot of times I either had a copy of the pages locally saved, or theoretically had the means to obtain a copy, so nothing was really lost; I figured that if the enwiki community ever unbanned me, I could recreate those articles using that content, or at worst I could create them on a third-party wiki. In the case of reversions, the reverted revisions remained in the page histories, so were even more readily available.
So, each time I got blocked, I would just come back and edit a different set of pages, which wasn't hard since I was always taking up new interests that were unrelated to the old ones. If someone accused me of sock-puppetry, it was because I quit caring about getting caught and therefore purposefully resumed editing the same pages I edited under a past account. However, since the last block, on 11 July 2012, I've almost completely abstained from editing enwiki (I suppose if I were at a library or someone else's house, I might anonymously correct a typo if I stumbled upon one). Originally, part of the reason for staying away was that I wanted to be able to honestly tell the ArbCom I'd fulfilled the conditions of the standard offer. However, from what I hear, the ArbCom isn't granting unbans pursuant to the standard offer much anymore; if they wouldn't grant it to Michaeldsuarez and PeterKa, whose alleged misbehavior was relatively mild, I doubt they're going to grant it to me. So, that consideration is now a moot point.
I can't put my finger on why it is that I don't edit enwiki or Wikimedia Meta-Wiki anymore, but it might simply be that I have similar sentiments to what MZMcBride expressed at a debate about an unrelated matter about which, having done no research to ascertain the merits, or lack thereof, of the arguments put forth, I can render no well-informed opinion, Meta-Wiki:Meta:Requests for bureaucratship/MZMcBride: "I'm much more disappointed in the . . . votes here from users I respect . . . . As I've watched this discussion unfold over the past day, I find myself wanting more and more to disengage . . . as I no longer want to be as closely associated with many of those who have become the regulars here. A wiki's culture is certainly expected to change over time, but it's becoming clearer to me that . . . the culture isn't improving. . . . This request was my attempt to volunteer to help out, but the vindictiveness, hypocrisy, and stupidity of some of those who have posted here make me question my overall involvement here." In my case, the disappointing votes were those cast by ArbCom members and certain of those who weighed in at a March 2014 request for help, but the principle is the same.
Some people will put up with a lot of what they regard as mistreatment on these wikis out of the belief that they shouldn't give in by letting people drive them out; they should instead try to turn the tide or at least do what good they can within the framework of the existing system and culture, flawed as it may be. I'm not sure what makes people finally change their minds and leave. It may be that other alternative forums and venues have opened up on the Internet for people to share information (e.g. Quora, third-party wikis, even Facebook), and so Wikipedia doesn't seem as important as it used to. On the other hand, Wikipedia remains a top-ten website and one of the most effective ways of sharing information with the world, for those who are willing to put up with the BS; the same usually can't be said of those alternatives.
It could also be that enwiki and meta's cultures have become less forgiving of mistakes, misbehavior, and even dissent, and the prospects of those cultures' becoming more tolerant in the future have become ever dimmer (due, I think, to Sanger's Law), so that there is less incentive to contribute. If you know that there is no path to redemption in which one could, by establishing a recent track record of good contributions, having the same opportunities as other editors for achieving respect and the other rewards of being an editor, then there is less incentive to come back. But I'm not sure how much people care about those rewards; wiki editing always was mostly thankless, but people did it anyway because it was addictive and they were obsessed.
Making what they regarded as improvements to articles produced a crack-like thrill of immediate gratification. What was it that made this stop being so appealing? I don't know. It might be that people got into a pattern of quitting and then relapsing, but now when they feel the urge to relapse, they pick an alternative place to contribute their knowledge (similar to how ex-smokers these days often turn to vaping or the patch), and it prevents them from getting hooked again. As they say, for the wiki addict one edit is too many and a million never enough; but if you can stop yourself from making that one edit that brings you back from your wiki break, then you won't get started down the slippery slope to a million.
One could say some of the same things about wiki retirement that one could about real-life suicide. There are a lot more suicidal threats than actual suicides, but currently there are rising suicide rates in the U.S. Maybe people are simply getting burnt out, but it's hard to tell. Even the people who are finally about to make that step over the ledge, after contemplating it for a long time, might not be able to say what makes this time different than all the other times they thought of doing it. Likewise, I can't really tell you why I finally left enwiki; I was disgruntled with the culture all along, and although it's gotten worse, there's still stuff I would regard as useful that could be accomplished by going back. For some reason I just don't feel like it, though.