I have read this entire thread, and came away with a few observations as well as a final thought / comment (albeit long).
Observation #1 : The legacy glass of the F-mount serves as a valid excuse to stay firmly anchored in the F-mount system.
Observation #2 : It is clear that Nikon still has the most compelling set of DSLR's as Sony abandoned their SLT efforts, Canon has moved on, and Pentax is dying. The four cameras... D500/D850/D5/D6 have been (and continue to be) the best in DSLR technology.
Observation #3 : There is a lot of money thrown at photography and pursuing tiny birds. The perceived need for the "fastest" autofocus seems to drive these discussion, while the motivation to produce artistically compelling images has become secondary to "best in tech."
( Note: Image boards reflect my comment in Observation #3, as photos with artistic merit seem to be less appreciated than those with frozen wings as a bird flits from one place to another.)
Final comment (I'll try to keep it short
)... Both mirrorless tech and DSLRs have there place in nature photography. There are clear advantages to both systems, and in some ways I think Nikon blew it with their D6. Nikon demonstrated the "power" of mirrorless integration into a DSLR w/ the D780, but they came up short with the DSLR specs on that camera (by not including the best DSLR AF spread). Had the D6 included the Z6/Z7 mirrorless tech in LiveView (per the D780), they would have really produced an amazing body. Short of that, it is an incremental upgrade on the D5.
Personally, I use my DSLRs with when speed is my priority, and I use a D500 because I abhor teleconverters.
Were I to do most of my shooting from a car, at national parks where photographers form a line to catch a bear/wolf/..., or at destinations where wildlife has accommodated to humans, DSLR's would meet my need into the future. Sadly, I don't live where wildlife is accessible, so I must hike into the woods, sit in a blind, or paddle to my destination. Low light defines my shooting period, and silence enhances my capacity to prolong a shoot. In these instances, mirrorless tech has created opportunities that would otherwise be for making images. In addition to silent shooting, low light AF, and in-camera histograms, I have found that the mirrorless tech brings out the best in my F-mount lenses. Some optics that struggle with consistency on my DSLRs (zoom or focus distance) are far more consistent because of the on-sensor AF system. Furthermore, when one picture is in focus, they are all in focus. Furthermore, lack of mirror-slap allows me to shoot at low shutter speeds and maintain acceptable detail.
While neither system is perfect, together they offer some larger that Sony can not match (legitimate access to legacy glass + future proofing as Nikon expands its system). At this point, only Canon competes with Nikon on the latter, but they have expanded faster. While Canon's R5/R6 are compelling, their DSLR's (save the 1DxIII) lack benefits of Nikon's system.
In the end one should make these decisions based on their needs rather than emotion... I often wonder if I made the right call splitting my efforts between time F & Z-bodies, fortunately Nikon maintained a consistent interface that allows me to switch cameras without thinking about the location of buttons, switches, and dials.
regards,
bruce