I would be willing to bet something substantial (not really) that the 600mm f4 will NOT have a built-in TC. But I am confident that its performance with the Z 1.4x teleconverter will be excellent, just as the current 600mm f4 model(s) are quite sharp when coupled with the TC14eiii. The price for the new 600mm with be at least $13,000, probably higher. This is the world we live in nowadays. I also now believe, sadly, that the 800mm f6.3 will be considerably more expensive than some of us originally thought. I bet at least $9000, probably $10,000. Sigh. Those of us who have used an 800mm lens know (I think) that for general bird photography this focal length presents challenges in terms of depth of field (shallow!) and actually being "overkill" for some situations, such as at feeders/blinds. Really, a 600mm f4 with the option of a 1.4x teleconverter is more versatile and with the 840mm f5.6 combo setup, optically just fine, thank you.
The reality is that in this world of increasing polarization between rich and poor (NOT trying to be political here, just stating the fact) there are LOTS of 'serious amateur' wildlife photographers who can and will shell out for these very, very expensive lenses. Wealthy, retirement phase baby boomers abound, with a "YOLO" mindset, i.e., you live once, so you want to take advantage of those golden years in which you (finally) have enough money to travel and take photos and yet you are still healthy and mobile enough to get around, travel, and walk and hike a bit. As someone in that demographic who recently developed an annoying (and possibly serious) arthritic condition in my left ankle that unpredictably makes long hikes quite painful, I have come to realize that there is a certain "get it while you can" aspect to all of this. Not to mention the realities of Covid-related travel issues.
I don't plan to buy any of these superduper expensive new long teles, but I understand that there are lots of people out there who will scrape up the money and make the investment(s).