[tedunangst]

Assuming the sandbox works. If the sandbox is porous, the attack surface balloons from apps I choose to install to every link I click.

[Ajedi32]

Not every link you click. Only sites that you grant access to the necessary attack surface. The Web USB API can't be attacked by sites that you haven't granted access to it.

[codedokode]

What if that privileged website has XSS vulnerability?

[Ajedi32]

Then the attacker gets access to that USB device. (And only that USB device.)

What if your unsandboxed native USB utility has an RCE vulnerability?