Fair, but remember that we are the <~1% of people who even know what webusb is. I'm not sure I share your view on this.

Maybe an about:config switch to enable it would be enough to stop casuals from pwning their peripherals.

I’d be ok with an about:config switch, but given that many people will install anything, paste arbitrary text into terminals, and share their password/pin code with complete strangers for almost no reason, I think we need to stop making our tools less powerful in pursuit of an impossible goal.