Yah Bruce, Your reasoning makes sense following what little is know how Nikon have constructed and maintained the quality of the their current exotic, indeed infamous, telephotos in Otawari or maybe it is still located within the Sendai factory. I've read several accounts of how the top end E FL primes are hand assembled in Japan, and the 800 f5.6E FL has to be matched and tuned etc by an expert in tandem with its bespoke TC125.
Back in 2018 at a talk by a Nikon ambassador, he described some feedback from his visit to Nikon Japan, where he met and thanked the elderly gentleman who had assembled and quality-checked his 600 f4 E FL. He described how each lens can be traced by its Serial No to one of a very few highly experienced technicians who build these optics based on fulfilling global orders.
I suggest we need to consider that Nikon makes the 500 PF differently, otherwise the expert labour would push up costs: probably robotic tools etc, which can include instruments to validate optical quality in alignments etc. "All" the 800 PF has to do is match the optical quality of the 500 PF shot wide open, and equally perform well with both Z-TCs. These are not unreasonable expectations, and we can expect Nikon to do this, and arguable on the Z system they may well deliver an even better prime. To speculate further, another factor in cost effective production is how to scale up a 500 PF ( 106-237mm) to a 800 PF of 313 x140mm with sufficient chassis rigidity?
However, as we keep seeing don't underestimate Nikon engineering: perhaps casting a honeycombed MgAl chassis is feasible and cost effective (?) Speculating further (!), someone posted somewhere (maybe on NikonGear) that the 800 PF optical design may position the Phasefresnel element further back >> so it can be smaller diameter. Again, speculation extrapolating from on what we know about the 500 PF. We do know that already over 3 years ago, that robots had taken over
75% of the tasks in assembling the Z7, which entailed many intricate precision processes.
Anyways, it will be very interesting to see what eventuates next year. Somehow I have to find the means to pay the RRP, a loan if needs be. I have found I use 800mm a great deal, as not only birds are 'too far' for a 500 or 600, let alone 400, but mammals are shy or simply too far from a vehicle/hide where a closer stalk is impossible. I also like to seek out tight framing of subjects in addition to shooting animal-scapes and environ-scapes.