> AFAIK you could, today, with no legal changes, have a vendor release 100% of the code under eg. a MIT license while also making the device refuse to run firmware not signed with their keys.

This is already the case today with many embedded devices. They have secure boot enabled so even if the vendor releases the GPL source code (big if), you can't do anything because the device will only boot the vendor's signed firmware.

> at a minimum I think there should be a wifi card that does refuse modifications and a main application processor that is 100% user controlled so that they can actually fix problems without needing the vendor to help

This is already possible. The RF components frequently have a signed firmware blob that is verified on load. There is no reason but planned obsolescence and greed keeping the application processor locked to running the vendor's signed code.