The main issue in the former case is that google is posing itself as a gatekeeper instead of following a repo model like Debian or FreeBSD. That’s wanting control over people’s device.
Allowing USB access is just asking to break the browser sandbox, by equating the browser with the operating system.
"I know what I'm doing, and giving a random website access to my USB host is the right thing to do."
"I'm an idiot."
How is not implementing a Draft spec, which may compromise security badly, breaking computing?
Overreacting much?
However in this particular case, even the security argument doesn't hold, either I:
a) know that I want to use USB - in that case I'll switch browsers or download a native binary (even more unsafe), it's not that I'd decide that I no longer want to flash my smartphone
b) I don't understand what's happening but I follow arbitrary instructions anyway - WebUSB changes nothing.
A 0day in a browser for the WebUSB system would allow any website to mess with arbitrary USB devices connected to your computer.
While the browser sandbox is generally safe, it is also a huge target, and with a security risk like that, it wouldn't surprise me if it's a prime target for black hats.
Maybe an about:config switch to enable it would be enough to stop casuals from pwning their peripherals.
So maybe don't populate the browser with dozens of features requiring permission popups?