The mirror is an obstacle right now.
For some applications, yes, such as video, or high frame rate stills photography.
It forces a certain flange depth sure,
Personally I'm in favour of mirrorless when it concerns opening up the mount, as I like large aperture lenses and also tilt/shift lenses, and eventually those would be advantaged by the new mount.
But for my people photography, all the EVFs I've tried have been non-starters. I hope there will be improvement but given the reported approximately 1/15s read time for the sensor it doesn't sound like it will be in this generation. A faster read sensor means shorter EVF lag but then there is a dynamic range loss at base ISO, so it is looking like most EVFs continue to be slightly laggy. This may not be an issue for everyone but it is an issue for some.
Reading directly from the sensor has many advantages and eliminates the problems of a moving mirror (mirror slap, oil spots, misaligned focus, blackout, weight, timing limitations and potentially cost).
I suspect the cost of the development of the technology that supplants it may well exceed the cost of the mirror and its associated mechanisms. Integrated circuits are cheap only when there are enormous volumes (e.g. 100 million copies, not 100 thousand like is typical in ILC cameras) involved and even then they are a real cost that is transmitted to the consumer.
The cost might not even be paid with a single camera but the expectation seems to be that users need to go through a series of progressively more mature generations to get finally what they want, but each time thousands of dollars exchange hands. That's the cost of mirrorless. In addition there is the cost of new autofocus lenses with new types of motors which make mirrorless AF possible. With DSLRs, requirements for AF motors are a bit different and most existing lenses were built for those requirements.
Mirror slap creates some sound and vibration, the vibration is mainly a factor when working at slow-ish shutter speeds. Here's my thinking: at fast speeds I am getting very sharp results even with mirror in normal mode and mechanical shutter in use. Moving subjects are frozen in time. If I need to shoot at a slower speed, and if I'm using a lightweight telephoto such as the 300 PF, I switch the D850 to Qc mode which slows down the mirror and opens the shutter early to let the vibrations die and with electronic first curtain I can get a sharper image in those circumstances. But for true sharpness and liberty from any vibration, I prefer a tripod as along with it comes not only freedom from vibration, also more precise compositional control and the photographs that result tend to be with much higher keeper rate and also more carefully considered. Especially with longer telephoto lenses I find that it's difficult to hand hold and obtain precise compositions. If I'm using a tripod I have a bag tricks (EFCS, silent shutter, mirror lockup, back LCD etc.) to avoid shake. I typically shoot my static subjects on tripod and moving subjects hand held.
For silent photography I am interested in a mirrorless camera, but I would prefer a model with an optical viewfinder, even if it means it is limited to a couple of focal lengths. However, I'm not very happy with the present implementation of silent shutter as it can distort moving subjects and also there is possibility of interference with the frequency of flickering lights. Thus there is limited gains in this area unless going to something like the A9, in which case silent shutter can be used fairly liberally but the same camera isn't ideal for other applications such as landscape. If Nikon can make this kind of a camera, it is of interest to me for situations where silence is a significant benefit. At the same time my belief is that the photographer's presence and actions cause at least as much disturbance than clicking a few shots on Qc mode on a DSLR. And flickering lights are very common; to me it is a delight that now DSLRs are able to time exposures to the peak of the flickering light cycle, which leads to more consistent results in such conditions (which are unfortunately very common). I would not want to go into a situation where I have more problems with these lights, banding etc. So I remain uncertain about the topic of the silent shutter. I don't normally shoot high fps, except for some subjects such as samba, and prefer my own timing based on the evolution of the subject in the viewfinder. I agree that some moments are easier to catch with high fps but I intensely dislike the resulting large quantities of data and the difficult and time-consuming process of choosing the best frame of many. I get much more satisfaction from shooting mostly in single shots and find the keeper rate is much higher that way as well. But that means the viewfinder experience has to be pleasing and with as little disturbance as possible. I look forward to testing Nikon's Z viewfinder to see where we are at the moment. In the A9 viewfinder I didn't like how it responds to flickering lights and would not be interested in paying money for that.
The DSLR is close to its peak as a technology so it is hard to beat, but I’m confident it will get beaten. Not this year, but soon enough. You can tell it is happening ecause the arguments are so fierce. Change is hard.
I think the fierce arguments are mostly a sign of the times (in society, humanity etc.) a result of liberating communication via the internet and positioning unverified hearsay at the same level as verified fact. Because people play it like it was a game, and strongly opinionated statements are prized with "likes", the discussion becomes polarized and heated. I do not believe this has anything to do with mirrorless or DSLRs per se. Also the Df generated a lot of heated arguments and it was just about how camera controls are organized basically, some people love the Df controls and others present heated arguments against them. Few people participating in the discussions seem to accept that it is subjective and everyone should have the right to choose for themselves. In the mirrorless vs. DSLR debate also people should accept that the answer is not the same for everyone, and diversity is a good thing, and benefits creativity. It's not a game with winners and losers. If diversity is lost then photography has become less rich.
