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?