I use my current ATV 4k 64 GB to stream self created timelapse movies shot with a DSLR from my NAS (Network Attached Storage) in the cellar to my 4K TV in the living room. If you render these movies in Final Cut Pro X, the maximum output quality the 1 GB Ethernet can handle is a medium ProRes RAW movie file. Better quality leads to dropped frames and the movie does not play seamlessly.
I guess, a Mac Mini now available with 10 GB Ethernet could solve the problem, but I don't want to sit in the living room with a keyboard and a touch pad - looks too much like everyday work

Sure, the ATV is not cheap, but I would pay an even higher price for - let's call it an ATV Pro - if it only satisfies my needs. If anybody knows a better streaming device than the ATV (app store with Infuse, VLC, Elmedia ...), please let me know.
So yes, I saw, the new ATV has HDMI 2.1, but the real "power" would only be unleashed, if not only the output channel is fast, but also the input side (and the internal processing too). One could argue, it has WiFi 6, but a fast Ethernet connection is much more stable and reliable than WiFi under full load.
Apple has all the technologies needed for such a device, but it seems, they don't see a market for it. Perhaps, they are right. Bad luck for me ...