I've been going back and forth between D500 and D850 for quite some time so all posts are of interest. Do you think there will be a shift with the release of the mirrorless Z6 and Z7?
I haven't seen any mention of housing for either of those, but then again this may be in the works. I guess the question you can ask yourself is what shift there was with the Sony A7 series for example, as they're mirrorless full frame as well.
There are full frame adaptors for the new mount with the Z6/7 I believe so you can re-use your old lenses, should they be suitable for underwater.
What I would consider would be:
* Battery life
* Lenses and compatability
* Housing availability
* Size, is it really that much smaller than a crop DSLR? possibly..?
* Heat, this isn't a big issue, but I believe mirrorless can increase fogging somewhat due to the heat they generate from powering the viewfinder/screen - but I guess if you keep your silca packs and your housing clean this isn't such an issue.
I'm sure if you looked at the mirrorless of today vs DLSRs of today, and you have your criteria, you could probably make up your mind now, assuming the Z6 and Z7 doesn't bring anything truly innovative to the table which UW photographers can benefit from.
There is no doubt that there are plenty of lenses that work well on land, but there are far fewer that give acceptable results behind a dome port.
For example, the only Nikon rectilinear wide angle lens that works behind a (9") port with full frame cameras is the 16-35mm f/4. It is not the best lens optically, so whether it will give acceptable results remains to be seen. Likewise the 60mm f2.8 Micro is not the best optically either. Lastly, and this is a big uncertainty, I am worried about the performance of the Sigma 15mm and Nikon 16mm fisheyes. Until the lens/camera/port combination is shot underwater, all bets are off and land testing has no relevance to this whatsoever. I found that performance with Kenko Teleconverters was unacceptable in terms of IQ with the D810 (this definitely impact on versatility.) The upside is that I would imagine that the new 8-15mm f/3.5-4.5 fisheye will definitely be OK.
I'm not sure where you are getting your information from, but the D810 dramatically showed the flaws of many lenses (e.g.17-35mm) both underwater and topside. The 5DS and 5DSr have also dramatically shown the issues with older lenses. In the case of Canon, they have admitted it...
I would absolutely agree that compared to the D300s - which is pretty long in the tooth now - it will be capable of images and creative techniques that would not have been possible with your older camera. However, I also feel that, for underwater use, I remain very unconvinced about it, especially when compared with the D500.
As you say, ultimately the proof is in the pudding. I'm not saying one is better than the other, but the D500 has been tried and tested underwater with great results, whilst the D850 has not.