How do you make sure that technically illiterate people don't just click away the requestDevice() popup? IMHO a browser offering device level USB access is a security nightmare and there is no way this can ever be made safe and convenient at the same time.
You can ask them to type one of the following sentences:
"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."
You simply don't. This quest of saving idiots from themselves is not gaining anyone anything and meanwhile other people get more and more useless restrictions.
They can click everything away, so maybe educate them or buy an ios device for your relatives instead of breaking computing for everyone else.
Isn't that the same excuse Gooogle is using to lrevent folks from installing what they want on Android phones?