Sigh.
Once you understand how OEM keys are managed and how sealed devices work with keys (laptops and tablets) it makes perfect sense.
For every pre installed device with Windows on it there is now a key in the UEFI BIOS. This identifies the device. That key will be unique to that device forever, so as you upgrade Windows that key is carried with it. The key no longer identifies a Windows version, it identifies the device.
This will be the case for probably 90% of every consumer device out there that gets Windows 10.
There are 2 exceptions. System builder OEM keys, where it is still limited to that device just like always, but the key is entered manually rather than stored in the BIOS. And then retail. Retail is the one people are getting all worked up about for no reason. Retail is not limited to a single device (although it is limited to only 1 install at a time) so retail keys updated to 10 will remain retail and therefore transferable.
So yes, it will mean limited to that motherboard for all devices where Windows is already limited to that motherboard anyway. Retail keys are the exception, but they are such a small fraction of the total market they haven't bothered to explicitly point that out. Much like they havent specifically pointed out how Business licenses will work.
Stop panicing. Do you have a retail version your planning to update?