When folks at Lens Rentals get a chance to open one, they usually do a good assessment. They know from experience what breaks easily vs. what lasts a while.
The Z8 uses the hybrid body approach they've used on pretty much all but the big, vertical grip pro cameras: a combination of metal and Sereebo (carbon fiber infused polymer). All of the parts that need alignment on the Z8 are mounted to a metal frame, the rest of the body is the carbon fiber type material.
In terms of "build quality," I'd judge the Z8 to be just fine, and probably more hardy than the D700, actually (that all metal frame has issues of its own, which can make some damage unrepairable).
I'm not sure I agree with either comment. First, take a look at any Z9 tear down and you'll see that the digital board doesn't extend down into the lower part of the body, because the battery chamber is down there. While I expect to see a slightly different digital board in the Z8, it's size will be quite close to that of the Z9 board. The big differences will be the different card slots and connectors that are attached. If I'm correct about this, the engineering required for such a repurpose is relatively simple and easy to do.
As for heat issues, I'd expect them to do basically the same thing they did with the Z9, which is to put a heatsink frame over the digital board. People think that because the Z9 body is so big that it has a lot of "air space" inside in which to dissipate heat. That's not really true. Again, look at any tear down. Will there be heat issues with the Z8 with the smaller body design? Maybe, but I suspect that will be more due to the fact that there isn't a metal frame throughout the camera.
You mean like the one they publicly demonstrated from Tower Semi? ;~) It's abundantly clear that Nikon has been doing stacked sensor work with multiple partners for some time. To date we've only see one appear in a product.
Going by patents filed I agree Nikon is continuing research into sensors.
But I am not aware of a joint demonstration of sensor design with Tower Semi. Please would you share a reference to that?
Nikon did talk about a 1-inch high speed stacked sensor, but as far as we know, that's just research and not in any sensor product in the market.
Obviously no one can know what prototype sensors Nikon has access to.
As I've noted elsewhere, the heat sink over the digital board on a Z9 essentially connects to the magnesium frame area just under the Rear LCD. This is a double-edged sword. Yes, it disperses heat from the sink, but it also is retaining the heat within the body itself. I believe that the biggest benefit of the Z9 design is actually the battery chamber having heat dissipation via the frame. The parts that heat up most during 8K/60P are image sensor, EXPEED7, CFe card, and battery. The Z9 improves that for everything over the Z8, but mostly the card and battery. Note that the card and battery are close together in the Z8. I'll bet that is the true gating issue.
Concerning the durability for the D700, I have one data point to share. Back in 2009, I took my D700 to South Georgia Island in the Antarctic. On a windy day, I unwisely set up my D700 mounted to a 70-200/2.8 AF-S VR (version 1) on a fully extended tripod. Not sure why I didn't pay attention to the wind and not holding onto that set up. Of course it took no time for the wind to blow the whole thing over. The body and the lens hit hard rock. There is now a permanent dent on the side of the D700, but the 70-200 had no apparent damage. Both continued to work fine for the rest of that trip.
Two months later, at home I noticed that the 70-200 had manual focus issues, but AF was totally fine and that was why I didn't even notice the issue. Nikon wanted $550 to $650 to fix the helicoid and I declined. Instead, I used that money towards a 70-200/2.8 AF-S VR II, as I knew version one had a lot of edge quality issues at 200mm.
I still own that D700 today and it is still working fine. Of course, if a different part of the body hit the ground, the outcome could have been very different.