Google, Facebook, and SpaceX have all moved to initiate programs to install global high speed wifi access.
This is probably the last edge chance for cellular network companies in the long term. If wifi hits first, with projects like Google home offering free calling as well as other ip-phone services I think we could see those companies begin to fall.
I'm taking the rather long view, though.
For average, non-power users, I could see these being quite popular -- especially if they can handle Windows 10, MS Office, some light Photoshop, and fast internet. Otherwise I wouldn't mind one to turn into a highly portable Linux machine -- stars in my eyes, and all that.
Tethering is hardly a second thought for most people who are comfortable with the technology, but you're forgetting most of the rest of the world here, as well as a lot of people outside the tech realm. Never mind people who currently live outside of high-speed wired internet zones -- with cell coverage reaching further than wired zones lately I'd consider the option a boon to connectivity.
I fail to see the real downside to the public. Maybe you can elucidate further.
If its in one package, and its as easy as hitting "connect", I think it will sell. That and consider cottage-goers who dislike disconnecting, or people who live outside of highspeed zones. Or even the connectivity-paranoid (script kiddies aren't so likely to start hijacking cellular connections as they are to bite you through an insecure coffeshop wifi). Doctors Without Borders might benefit, and other like travelling charity groups. I'm just spitballing, but I can see cases.
Though, a lot of my speculation depends on price points.
Do you mean the USB-C with the gazillion required adapters or new cables?
It's relevant you bring up Apple, because if you have a MacBook and an iPhone, and pair it via Bluetooth, the a tethering option shows up in your WiFi menu that you can easily select, same as any other WiFi network.
The advantage of it being an easy, yet manual process is that you can't accidentally use it and blow through your data. I do wish they would go the extra step and build "limit what you do because you're on a metered connection" support into macOS.
The difficulty for average users is the knowledge that you can tether, setting it up, and remembering to turn it off. The UI's are good at reminding you, but even I've forgotten if I've had to run off the train because I got caught up in work.
Metering (with overrides) would help absolutely!
Also, how can you limit data usage while tethering? Can a device tell if it's tethered, or does it just assume it has a normal wifi connection? Do you need to turn off automatic updates on laptops?
Though Windows 10 does let you mark a connection as metered, though I don't think anything except the OS pays attention to that. But again, that's orthogonal to whether your laptop has a modem and will likely not be solved.
Assuming you'll be able to boot Linux on such a device, that might still require exploiting some yet-to-be-found bugs to root it, and/or voiding its warranty. BTW good luck with GPU drivers, multitouch, exotic sensors...
Whether they take off or not will depend on many factors besides LTE. Those include size, weight, battery life, performance, Win32 compatibility, marketing, advertising, availability and price.