A WiFi network in a controlled environment is one thing, out in the real world is a whole lot different.
The network adapter for the D4, the WT-5A Wireless Transmitter is a dedicated device costing £480+ if you can find one to buy. The reason for the price is the range and reliability. I very much doubt Nikon have ben able to but the same quality and range into their bodies. And also the ability to connect to computers.
Ditto another option is the WT-7 wireless transmitter, which offer more feature and is over £1000. Again, do you think they have put this technology into a camera.
Now they also do a Nikon WU-1b Wireless Mobile Adaptor for about £65 that lets you establish a wireless connection between specific Nikon digital SLRs and a smart mobile device. Now that sounds like what I have built into my D500, and what these Z cameras may have, but, the range is limited and Nikon have limited access to their WiFi to their app, though I think I read they may have opened it up a bit on these new cameras. Another but, are you going to be the guinea pig to try this out in the stress of a wedding. Taking images in a studio is one thing, the real world is a whole lot different.
And what is the problem with having two cards and WiFi back up! You just increased your redundancy, though probably drastically reduced your battery performance.
It is amazing all over the internet how people are making excuses for these flawed design choices. Those that need two cards or want two cards know who they are, and for a lot of them this may mean they don't buy these cameras, or maybe just one when they were considering more. Those that don't need two cards, or do but are willing to let it slide because it is Nikon, those who don't see the potential problems continue to make excuses, for what reason I don't know, it doesn't matter to you, but at least see the point of view of people for whom it does.
And as for the choice of card for these cameras, other than 4k video, which other cameras do quite well on fast SD cards, these cameras so far are not pushing the XQD performance limits in any way. The buffer is small, and does not clear quickly from some reports, even with Jpegs. On my D500 I could take 200 lossless 12bit RAW files, lift the finger off the shutter button and do 200 more. These cameras are severely limited in buffer size, and I'm fearing clearing the buffer, when the D500 proves that that shouldn't ever be a problem with XQD, especially with the small number of fps.