I've definitely eyed those as well as the small Sony FFs, but making the noble sacrifice of sticking with the Z5/6/7 weight class due to longtime affection for Nikon and their cameras, colors, interface, lenses.
I'm not going to dig through all my files and bring up the evidence we used against Nikon to get remediation for D800 owners. But what that showed was a change in the metal frame on the D810 at exactly the point that failed on the D800. The plastic material used in the "mirror box" is not precise enough to guarantee focus sensor array alignment. That's actually been a point that people are arguing with me about on the Z8: that the Sereebo material isn't precise enough for tight alignment. And the focus sensor array on a DSLR is the tightest alignment on the camera.
Whatever Thom, D810 mirror boxes are plastic. All of them. D800 ones are magnesium.
Look here to show you a bit more. Of you could look at Roger Cicala's teardown of a D81, and you can see that the whole internal chassis (frame) is plastic. The question I have is why Nikon changed from the magnesium in the D800. BTW, from the first one you can see that the AF unit is located by metal stand-offs. Presumably these are shimmed to provide the required alignment.
Presumably Nikon found a way to make it precise enough. 'Bad pours' cannot have had anything to do with it. These parts are thixoformed, there is no 'pouring' involved.
So far as the Z8 goes, this is another case of concentration on the materials rather than design principles. The key stresses that need to be managed in a camera are the stress between lens and sensor, the stress between the hands holding the camera and the lens-sensor combination and the stress between the tripod mount and lens/sensor. In a SLR all of these go through the mirror box, which is inherently a weak component, a big box with lots of holes in it. None theless, the majority of DSLRs operated fine with plastic mirror boxes (including high-price items such as the Canon 5D series and, as above, the D810). Nikon's 'monocoque' structure (D5500+, D750+, D500, D7500) changed that a bit by building a structural front panel (in Sereebo), which took the loads from hands and tripod mount to lens mount, leaving the mirror box only taking the load from lens mount to sensor, so it bore very little stress. Mirrorless cameras don't have a mirror box, so tend to be designed like a classic Leica, a structural flat tube with a lens mount on the front and the sensor attached to the back of the lens mount using a spacing piece that isn't structural. The Z8 looks to be much the same, except that the 'flat tube' is Sereebo and the spacing piece (and front panel) is mag alloy. Apart from materials the design has no commonality with the D810 - in fact it's almost a reverse. What's plastic in the D810 is mag alloy in the Z8 and what's mag alloy in the D810 is plastic in the Z8.
So far as 'precise enough', neither plastic nor thixoformed mag alloy are 'precise enough' for lens-sensor alignment. The mag alloy needs post machining to bring it into tolerance, plastic mouldings are usually shimmed to do the same. Take the lens mount off a camera and you'll usually find shims to deal with that.
I was going to snip some of your post so as not to fill up the page as much, but didn't know where to cut. Now adding length by explaining that.
There may be no logic to why Nikon keeps switching materials around.
I run high performance snowmobiles. I made a comment on DPReview a number of times that Nikon seems to design and manufacture like the snowmobile manufacturers do.
In any given year they may re-design the angle of the front A arms, change the approach angle of the track, take out or add wheels from the skid frame, lighten the machine by going to thinner or different materials, change chain drive to some type of direct drive, make seats smaller, change internal engine parts to either lighten the engine or beef up the engine. Dozens and dozens of changes every year. Yet you still can buy a third party "durability" kit for 800 Polaris engines so they don't go all to hell, you can buy wheels to go in the skid where the manufacturers left them out, you can buy third party bumpers as the manufacturer put lighter ones on to reduce weight, and on and on and on.
I would not be surprised if in any given year some engineer at Nikon doesn't overrule someone else and say we are using metal this year, or we are using plastic this year. (metal/plastic for simplicity) Absolutely nothing says one will be better than the other, it can be done on a whim, we'll give this a shot.
My buddy and I got to test a pre-release snowmobile one particular year. I found it to over-steer, it was front heavy, the rear suspension was not compliant. The fellow that provided the machine was all insulted with our feedback, but after release the particular model turned out to be a joke. As I said about the snowmobile manufacturers, no one ever tested what they built.
Camera building is likely just as bad. No one "tested" the D800 before release nor the Z6 or Z7. It was all throw it out there and hope no one notices, pretty much identical to snowmobiles.
So likely a waste of time to wonder why metal one year, why plastic another.
Indeed. Of more concern is the performance of a particular material in the intended use. Just saying "plastic=bad, metal=good" ignores that consideration.
I think there are likely individual engineering teams that do things in their own way - so if the Z8 and Z9 had different teams doing the body structure, they could each have decided to go their own way. In the case of the D800 to D810, it was an 's' upgrade, so the default case would be no changes unless needed for a new feature or fixing a problem. That's why it's likely that the change from magnesium to plastic was part of fixing a problem. One possibility I just thought of is that the plastic, being more flexible, might have protected the fragile bit from impact when the camera was dropped or knocked. The interesting thing is that the lens mount screws into the plastic bit, so it would have that affect. In the case of the Z8, I suspect it's a bit of feature engineering. Somehow Nikon has to justify the $1500 list price difference, and that is part of it. Another possibility is that if you look at Roger Cicala's teardown, you see that the Canon R5 is built the same way - so maybe the thinking is just 'match the competition'.
Just throwing in my 2 cents worth.
Perhaps the primary reason has to do with mass automated manufacturing which Nikon has seemingly spent a great deal in transitioning to.
It may be possible that whatever mechanical thresholds Nikon has set has been met by either materials in whatever combination but a certain combo lends itself better to their mass manufacturing techniques so that gets the tick.
This may be the a new body platform for forthcoming models including maybe a higher end DX and maybe the Z6 III and/or Z7 III.