David, whilst i'm not doubting that the Z6 returns much higher quality files than my OMD system, there's just something about using my Olympus cameras that makes me want to use them - much more than my Fuji gear ever did (I think in hindsight, that was an itch I just wanted to scratch after hearing all the magical things about the Fuji sensors and lenses). For me though the great advantage of the "pro" lenses is that in terms of focal length equivalency, they are still generally much smaller. Indeed, in my day pack today (which goes everywhere with me), is my OMD-EM1 MK II, M.Zuiko 12-100 F4, M.Zuiko 12-40 F2.8, M.Zuiko 17mm F1.8, Leica 8-18 F2.8-4 and my Panasonic 100-300 F4-5.6 II. So in a bag that barely weighs 4 kg I have a pro body, 5 lenses with FL's from 16-600mm equivalent and all my other "day" stuff (chargers, keys, mobiles, power bank, batteries etc.)
Try doing that with FF - even the Z6 which I absolutely love. It get's even greater when I'm shooting wildlife. I can take the same day pack (just re-arranged internally) with the EM1 MK II or the EM1X and either the 300mm F4 IS (with 1.4x Tele) - so 600mm (or 840mm with converter) or the same body with the Panasonic 100-400 (200-800 equiv). When I recently had my D500 with grip and the 200-500 (so 300-750 Equiv), whilst the ultimate high ISO performance was a little better than the Olympus, it was a beast of a combination and weighed a ton. So much so that I hardy took it out which is why I sold it - and that was APS-C - not even FF).
So whilst the FF bodies have reduced dramatically in size (the Z6 is almost identical to my EM1 MK II, and much smaller than the Panasonic G9 I used to have), the lenses (as a result of pure physics) just haven't' (yet). Interestingly Nikon appears to be a bit of a leader in the field of getting FF lenses smaller with the collapsible 24-70 and 14-30 units and the use of Phase Fresnel elements in some of their tele lenses reducing the size of them down considerably.
For the time being then, having Nikon mirrorless FF and Micro Four thirds makes total sense to me.