GPU vendors have come to the realization that the in-kernel driver needs to be open-source, but the userspace portion can be closed-source. There's just really no good reason to accept a design where outdated closed-source drivers could keep you from running a current kernel. WiFi NIC vendors have for generations been moving more complexity into the closed-source firmware blob that runs on the NIC's own processor core(s), so there's no good reason for the kernel driver to remain closed-source.

Broadcom has been doing FullMAC designs for over a decade now, and that is exactly what you describe: moving all the functionality into firmware and having thin opensource kernel driver