UEFI and "secure boot"

Agreed. Many big Android device makers have announced that in the future their bootloaders will be unlocked (worth a quick Google). The carriers wanted lockin but the customers are saying no.

Non-Apple customers that is...

It is going to depend a lot on the vendor and it may not get off the ground, we've been talking about general lockdown using the TPM for many years but it has yet to come to pass.

I would imagine that if this does become popular that it will be used mostly by big vendors who sell low end devices for home use, and Apple. Since the computer will not be entirely user-modifyable its more like a rental or a service. This could be used for appliance like systems where all maintenance is done by the vendor and only vendor-approved software, drivers are used. There is certainly a market for that kind of thing, as Apple is demonstrating.

In the highly unlikely event that general consumer hardware becomes widely keyed to only boot Windows that might actually boost Linux specific hardware vendors like System76 or Dell's N-Series lines as there is market demand for non-Windows systems but regular computers run non-Windows well enough that there isn't much of a market for systems built for non-Windows use.

Motherboard vendors will never pre-key such a thing as their customers are far more diverse, technical and demanding. Motherboard vendors don't necessarily cater to or test non-Windows use cases, they aren't going to go out of their way to alienate those customers either.

A lot of this TPM and UEFI tech makes sense if it is just set up during initial system install or first boot and is under control of the owner.