You keep saying that F mount lenses will not peform as well AF wise on the Z compared to an F mount camera but:
- a large majority of hands on report claim just the opposite such as this one: https://youtu.be/t-nGejFGlO8 where the notoriously slow focusing 70-200 f4 is used successfully to shoot mountain bikers in a forest
And there are just as many saying it's slow and sluggish, and not up to the standard of either Nikon SLRs or Sony A7III/A7r3 bodies.
Out of all complaints in Z6/Z7 reviews so far, complaints about autofocus performance seems to be by far the most common. I think Tony Northrup compared it with the A7r2 - a generation and a huge gulf in capability behind the current generation.
- there are other advantages such as better coverage and on sensor AF that will remove the need to calibrate lenses.
None of which matter if the lens/AF system can't keep up in the first place.
I don't need AF to achieve perfect focus on nonmoving objects - I can do that even more accurately with manual focus. I need AF to track things that move. Going on reviews, it seems the Z6/Z7 don't do that as well as either SLRs or Sony bodies.
Now, back on the Canon, is the value of the mirrorless R camera and lenses for a Canon current DSLR user more clear for you than the value of the Z for a D850 user?
Yes. Largely because of one thing - the 28-70/2. It only applies to a subset of users (event photographers), but it is a significant subset - far larger than the pro sports or wildlife supertele users, or the artsy types who might want a manual focus 58/0.95 - but, for those, the 28-70/2 on mirrorless body with Canon flash system represents a significant new capability that wasn't available on SLR. Also, for entry level full frame users, or those looking for a single lens/body solution, the mirrorless body with 24-105 looks to be better in almost every respect than the 6D2/24-105 SLR combo (the 5D4 being in a completely different price category), with a leg up in resolution and low-light AF over the Sony competition (although losing out on the dual slots).
In the case of Z7 vs D850, there's basically nothing other than video that the Z7 can do that the D850 doesn't do better.