The security implications if this goes mainstream is that you are expected to do this for all kinds of hardware.

Right now that isn't the case and I can't remember last the time I had to uninstall untrustworthy native drivers.

A lot to lose, very little to gain?

I felt that way too, but having used it a few devices as an end user I enjoy being able to close the browser and have the whole stack disappear. Instead of having to install a creepy Logitech tool to pair a mouse with a receiver, as soon as that task is done, goodbye Logitech. I guess a real concern is manufacturers stop offering native drivers, but for the majority of hardware the PnP or the Linux kernel just handle it.

There's a real risk of losing the ability to control your device if the manufacturer stops hosting their propertiary WebUSB app, too.

Standard USB drivers aren't going to disappear from my disk and can be reverse engineered long after its manufacturer has dropped support or gone under.

So what is an example use case where you'd prefer to do X without using this particular tech?