Why should we build this functionality into the browser? Is it really that much more convenient than installing an app to run the proxy?
The use case you describe is sufficiently rare, that if you need it, downloading an app should not be a high cost to pay. If only 1% of browser users need this USB proxy, why expose an additional attack surface for the 99% who do not need it?
There could be something similar for USB, where there might not even be any "codecs" (drivers) included by default in browsers, but where there's a defined method of making USB devices available.
You'd still have to install a plugin for each device type that your browser doesn't natively understand, but standardization for how to make specific device types available to the browser would be much simpler. And there could be "rewriting proxy" drivers available for improved security which limits what requests that are possible, preventing most hardware exploits.
Although this might still be too much work for little gain.
Apparently it is, given how highly requested of a feature it is to have everything you could do with VMRC available through the web browser with no plugins or local software.
Again, I still don't think it's a good idea either, but I also don't like a lot of the crazy shit built into modern web browsers for maybe the 1% users, like the mentioned WebMIDI.
Unless you want to connect your local USB device to some VM in the cloud. Which is the direct use case that came up when this came up in discussion with Reilly Grant when we were both at VMware. It comes up surprisingly often as a requested feature, as IT organizations get increasingly lazy/less funded/laid off and don't want to have to install clients or anything on local machines, just pop open a browser, point it at the VM and connect up the device.
Not that I'm advocating for this in any way... I thought it was pretty out there (along with WebMIDI), but hey, why not, the Web Browser is the One True Platform and everyone and their parent company and sister company has nearly quit doing native application development on any OS (as sad as that makes me).