[setr]

>If C receptacle to A plug adapters exist, they allow the user to plug one in on both ends of the cable, creating an invalid cable that devolves into a USB A-to-A cable. This is why the specification specifically forbids all legacy adapters that have a C receptacle on one end.

I don't understand this statement; If an adapter for USB-C to USB-A exists, then the usage of it would force the USB-C operations to be limited to USB-A specs?

Isn't that exactly what you want? USB 2.0 is slower than 3.0, so interaction between the two devolves to USB 2.0; USB-A is slower than USB-C, so interaction devolves, and now you're backwards compatible (omitting whatever benefits of USB-C, but still a working setup nonetheless)

In fact, I'd imagine thats what adapters do in general.. if they're between non-equivalent things, it naturally devolves to the maximum commonly supported