Would I buy a mirrorless camera ? - Absolutely, once there is a system around that fully realizes the special advantages of mirrorless, without introducing artificial problems over DSLRs that dont need to be there in the first place.
Small and compact is NOT a priority of mine. The reasons why I would like to have a good mirrorless camera are:
1. Increased AF area, thus more freedom for composition
2. Shorter distance between sensor and lens, thus simplified design of wide angle lenses, thus better image quality in this area (also they are smaller)
3. EVF, which is a mixed bag of advantages and disadvantages over an OVF, but overall I think it gives me more advantages for improved quality of the final image:
3.1. Good confirmation of focus (focus peaking, magnification, digital split screen).
3.2. Better control over exact exposure (life histogram, zebras).
3.3. Better control over white balance.
3.4. 100% accurate framing.
3.5. No danger to my eyes when shooting into the sun.
4. The AF of mirrorless cameras can be accurate without the need for calibration.
All in all I want a system that solves problems that my current DSLR setup doesnt solve yet. Without losing advantages I already have and dont need to get lost.
Would I buy a Nikon mirrorless camera ? - Yes, if they offer the system that realizes the aforementioned advantages of a mirrorless system the best way.
I would love to get a Nikon mirrorless, because then there would be an adapter from the Nikon F to S (assuming they reuse their old rangefinder Nikon S mount) with autofocus and I can keep using my Nikon flash and flash system.
But right now Sony FE is the best option, and its pretty good already.
Would I buy a Nikon mirrorless camera that keeps using the F Mount ? - No, because that doesnt give me the advantages I want from mirrorless.
Do I think DSLRs will die out in the long run ? - Not even rangefinders have truely died out, one still can buy the Leica M.
The SLR concept has some strong advantages that wont die, such as no current needed unless a picture is actually taken, so I think they may persist in the long run.
Nikon sits currently at #2 in the camera business overall, right behind Canon. NEITHER company needs to make anything they do not want or wish to make, at this point in time; doing so would possibly only justify their competition and their efforts.
I do not care about that. Whoever gives me the best solution for making good pictures will win.
I wont be interested in an APS-C solution, because otherwise I would have switched to Fuji X long ago.