This is such a bittersweet situation for me personally. Nikon's DRF dot feature helps tremendously with focusing MF glass and without much interruption but it's not nearly as accurate as a magnified EVF view. But optical is optical, no getting around it. And native lens support, automatic aperture stopdown in particular, does enhance the satisfaction also.
Overall, not sure what I should make of the Df/Z6 situation.
I'm basically with Ray on this one, but what I really, really dig is the interoperability of the Nikon systems - I'll keep my D750 as long as it'll last (even though I've started shifting AF-S lenses in earnest - the Z mount counterparts outdo them easily) because I like the native F mount and optical viewfinder capabilities; and after acquiring the 28-105mm AF-D, I actually own a fun package to carry around, too. And I can use all of my F mount lenses with little to no penalty and without having to fiddle with magnification or anything. The AF(-D) stuff even autofocuses ... Of course, you can get technically better *results* with the EVF - but not better *images*.
The Z 6 provides everything else (small, stabilised, EVF, great glass, right down to the superzoom - heck, in most cases, the Z 24-200mm actually equals or beats the venerable AF-S 70-200 f/4G). And the FTZ adapter works fantastically well - which actually opened up my new tele solution for me: Z 50 with FTZ and AF-S 70-300mm f/4.5-5.6E - great combo; the lens works equally well with the D750 and Z 6, too; it's one of Nikon's true sleepers - inexpensive, very decent results.
However, I've really had to stop myself several times from adding an L mount body (i.e. the S5). I really, really like what's available for that mount - however, it doesn't add anything specific to my kit that's not already covered, at least not in the way I want, so I'd better stop thinking about it. Another system definitely wouldn't help things, anyway.
I do hope thtat 2021 will get me to the point where I can once again push out one or even more systems and still keep the capabilities (i.e. mostly lenses and use cases) they provide. Nikon and Sigma can make that happen. Sony will be the one to go first ... The E mount was (well, is) fun, but the cameras are *not* my cup of tea. They're not bad - I just don't like them much; the lenses I own for them is what makes me keep them. If only Sony (and maybe Samyang) would manage to crack the Z mount ...
M.