If the argument here is that microkernels have failed because they are incredibly successful in two very important niches, then I feel like you are applying a d

If the argument here is that microkernels have failed because they are incredibly successful in two very important niches, then I feel like you are applying a double standard. Ultimately, Linux also is not all that popular except in two niches: servers and cell phones. Windows is also not popular except in a niche: desktop computing. Ultimately, you have arbitrarily declared several very prevalent forms of computing to be 'niche', while declaring several equally prevalent forms to be 'popular'. One of my hypotheses above is that we tend to dismiss reliable forms of computing as 'not really computing', because we expect computers to fail. When computers work, we think of them as mechanical machines or devices (the way we might view eyeglasses for example). I feel like your subjective opinion of what is and isn't niche just further demonstrates this point. My guess is that there are more instances of QNX (and systems like it) than we realize. There could easily be as many as Windows or OS X (almost certainly the latter, I feel).