I'm excited for the potential here.
If Apple keeps the current chassis, they could either make it more silent, or more powerful. Both would be fantastic for a pro machine.
Edit: I wouldn't even mind a slightly larger machine. The HP Z2 Mini G4 looks like it's going to be a great six-core workstation.
You answered your own question.
I'm sure someone is using them for something, but I've never fretted how big a stationary computer is.
They also have somewhat better cooling than laptops because they're so thick, and you can buy silent passive cooling cases for them.
It's not like Apple's going to make the new Mac Mini three times the size it needs to be and stuff a bunch of 3.5" HDD bays in for people who want to use it as a file server.
Smaller with fewer components and everything soldered/glued together is a cheap way to build smaller and cheaper at the cost or repairability, and Apple clearly doesn't care about repairability. They've said that the Mac Pro redesign will be more modular, but I don't see them doing that here.
[1] https://www.reddit.com/r/intelnuc/comments/7ga2q9/skull_cany...
https://www.ifixit.com/Teardown/Apple+TV+4K+Teardown/97511#s...
At least since it's not a handheld touchscreen device, you can take it apart and replace stuff. It even uses Torx screws! But there's not a lot of discrete parts here, just the power supply,the motherboard, and the fan.
Once upon a time the Mac Minis would have let you replace the RAM or disk, but I'd expect it to look like a similar similar power supply + motherboard + fan in a box.
Minis a few years ago had a quad core processor option and a lot of people have been disappointed by its absence, making the current generation objectively worse than previous ones. I don’t think they’ll pass up the option to put it back in, IIRC it was something to do with their choice of Intel’s processor families that got it dropped in the first place.
https://www.newegg.com/Product/Product.aspx?Item=9SIAE4N7YU8...
And it could actually be "Mini", which is increasingly a stretch for the current design from 8 years ago. The Apple TV is probably smaller and more powerful if you could run a general purpose OS on it.