Flashing was already solved by UF2, where the device-to-flash temporarily pretends to be a USB storage device. Giving raw USB access to to random websites for that is massively overkill.

I could understand it if you were trying to do realtime configuration of or interaction with some device like a printer or a Stream Deck, but something as trivial as firmware flashing?

It's pretty common to pick a few config parameters, click, and flash a firmware that does the things you want.

Yes, you could make the configuration into a separate uf2 object that overwrites other bytes, but that's yucky.

The access is explicitly per device. Even for plain flashing, it's safer and simpler than to download and shuffle random files.

trivial for you maybe but many people don't know how and where to find the right firmware for their specific device, and can be in environments where the UF2 volume isn't as obvious (e.g. using a phone)