Interesting thread. It reminds me of a small group of caveman sitting around their fire and debating whether sabertooth tigers will replace the last dinosaurs. The truth is, technology has changed astoundingly simply in the last 2 or 3 years. Photographers who are much smarter than I am - and I include Thom Hogan in this group - have been predicting this for years. Sony's A7 series of cameras have redefined low-light photography, not just for mirrorless but for any cameras. Olympus's flagship E-M1 has redefined both EVF's and supposedly slower micro 4/3 focusing. Fuji's sensors and IQ's are powering generations of not merely advanced amateurs (like most of us) but a new group of pros who've gone entirely mirrorless and aren't looking back.
And, yes, there are certain niches where the older sabertooth tigers and dinosaurs of the photographic world, can still beat out the new-comers. But they are shrinking - and that, coupled with the size/weight advantages of mirrorless (which are very real for the majority of casual photographers, not necessarily 'us' but the market as a whole) and the iPhone/smartphone revolution (which has basically destroyed one of the largest market segments in digital photography overnight) - all of these are examples of "writing on the wall" which, now, is written in such large letters that if we can't see them....I'm afraid it's because our heads are buried in the metaphoric sands of time.
And yet...
There is still some truth to the old adage that - it's all about glass. It's all about lenses. Great lenses are what make great cameras and not vice versa. Going all the way back to the Leitz Wetzlar early days of Leica - to Zeiss's legendary optics - and coming up through modern times where Canon and Nikon aren't the only ones to manufacture great glass: don't forget the long and rich history of both Pentax and Olympus in that area as well. So maybe it's not mirrorless vs DSLR --- but rather glass vs glass - and as more and more lens makers start making truly quality optics for non-DSLR formats, another advantage of the dinosaurs goes by the wayside.
And no, we're not there yet. But we almost are. This stuff (technology) happens fast. Close your eyes for a second, blink - and everything's changed. Case in point: Sigma lenses. Over the past several years, Sigma has invested a lot of time, energy, and thoughtful innovation in making a series of great - and ridiculously affordable - lenses, not just for FF cameras, but for a variety of APS-C formats and most recently, the micro 4/3 mount. Many of these Sigma lenses aren't just 'good' - they're great. Thoughtful photographers like Ming Thein - who's posted several of the more thorough and interesting reviews of the latest 'super lens' the Zeiss Otus - pretty much admits that when push comes to shove, some of the high-end Sigmas are just as good. Roger Cicala, the guru over at lensrentals.com, pretty much says the same thing.
I like sitting around the campfire and debating the relative merits of sabertooths vs dinosaurs just as much as the next photographer, don't get me wrong. But in my own personal photography, after decades of shooting with superb analog cameras & lenses (mostly Leicas, Contaxes and Pentaxes) - and after transitioning to high-end Pentax DSLR gear for awhile (mostly because of some of those luscious and semi-affordable Pentax primes), I finally got sick of having to lug around pounds and pounds of camera body + lenses during a multiple hour hike. I've joined the ranks of mirrorless - with a superb (for what it is) Lumix GX7 (the favorite mirrorless of another great photographer, Carl Weese) - and also the hugely underrated Canon EOS-M which, with its ridiculous quality, and its remarkable (and remarkably tiny) 22mm pancake, and its upgraded firmware (not to mention the recent fire-sale prices, when Canon inexplicably dumped the EOS-M, for North American markets at least) may be one of the best semi-pocketable serious photographic tools available.
But sometimes those sabertooths can keep a person awake at night 