I wanted to give my view on all of this Z, mirrorless, DSLR...etc...
My fairly unbiased experience is based on this transition through the various systems....diehard Canon shooter from 2009 till 2015....still own some big glass...at one point had 6 Canon bodies in my house and even more big whites....bought a D500/200-500 in 2015 to try it out and shot it off and on alongside my Canon kit. I believe the D500 is far superior to the 7D2...but I continued to use my 5D4, 1DX, 1DX2 a lot while owning the D500 for the first year. In 2017 I started selling off more of my Canon kit and started to slowly expand my Nikon kit as I just found AF for fast, small BIF to be superior and was getting shots easier than with my Canon stuff. Bought 300PF, bought D850, downsized Canon eventually to just 1DX2 and 400DOII, 600II, 100-400II (and kept my normal/wide zooms..see sig). Then I got to try the A9 and A7R3 in April in Florida with my buddy who brought them on the trip and let me shoot them as much as I wanted...we rented the 500/4 Sigma and MC-11 and used the 100-400GM with both TCs as needed. I was very impressed by the A9...especially the e-shutter, blackout free, lag free, distortion free, and dead silence if you so choose....also very impressed with the AF and found it probably at least as good as Nikon and maybe better. Wasn't sure if I wanted to invest in the rather expensive A9/100-400GM when I returned home but just then found a local selling the whole kit (A9/100-400Gm/1.4TC) for a very reasonable price. Jumped on it and after more and more time found the A9 AF was a superior system in most situations to the Nikon and surely all situations to my Canons. I also could not get on with the A7R3's EVF experience with lag, blackout and difficult to track a BIF. Added a 500/4 E FL Nikon and sold my last Canon body and the 100-400II.
There are a lot of great systems out there and Canon is not totally lost...the 1DX/1DX2 is still an amazing piece of kit....actual AF speed is still faster on the Canon bodies but unfortunately the AF consistency is not up to Nikon or especially Sony A9 levels (the A9 is uncanny how many tack sharp shots you get....way too many if you shoot 20FPS...which is why I mainly keep it at 10FPS). Ergonomics is an issue with the A9...my fingers don't get pinched by the GM lens as they are rather long and on the skinny side but an unsupported pinky requires the battery grip but once the grip is on the ergo is really good for this small of a camera and I'm comfortable for an entire day. Without the grip or a plate I get finger/hand cramping fairly quickly with the big lens and also with smaller Canon adapted lenses. The Eye-Af on Sony is really amazing and I only use it with adapted 24-70/4IS and 16-35/4IS Canon lenses and still it grabs my nephews and nieces in tack sharp shots all the time. Adapted 400DOII works well for static shots and even for flight against sky but if you are way out of intended focus range it isn't pretty to get back to focus...prefocus in the range is needed. However once in the range it can track a bird bouncing around on a branch and nail tack sharp shot after shot just like the native 100-400GM.
These Nikon Z cameras are not for my intended use...they have some pros over Sony A7 series like ergo in the grip and availability of the F-mount lineup but they are of course more akin to the A7 and not A9 (as I always expected before release). However, they are lacking stuff compared to the Sony A7 already like Eye-AF, buffer, battery life (still to be determined and surprising since they take the D500/D850 battery), and AF at higher FPS has limitations...also in early FW the buttons aren't very customizable...unlike the D500/D850/D5 you can't have other AF modes/AF-On on other back buttons (like joystick push)...that is disappointing.
Unfortunately I had much higher expectation for the Z release and I really don't have much hope that Canon is even going to match the Z release and surely not come anywhere near the A7 series or of course the A9.
Therefore I'm buying a Nikon 500 f/5.6 PF as I love my 300PF but always have TCs stuck on it....I'm going to hold onto my 400DOII/600II and my zooms to see what may come out but my patience is running thin and if more cool gear comes out from Nikon and/or Sony I may start selling the Canon big guns to fund other stuff....PLEASE CANON...wake up and make us proud to be Canon shooters as I was back in 2012 when I got my 600II....