In order for this to be even vaguely secure, every single device that declares support for WebUSB has to refrain from allowing firmware updates over WebUSB despite how incredibly convenient this is, and despite how much of a support nightmare it would be to roll out bugfixes to users without it, and despite them being developed by hardware manufacturers who generally don't give a fuck about security. I can't see this happening. (Allowing remote firmware updates effectively gives full remote code execution on the host PC to the website behind the device, since it can then emulate USB input devices.)