I wonder how that will affect older devices, like Android TV streaming boxes, where you use the assistant through the remote.

I see that the article mentions this, but I'm skeptical. I think that the assistant will just be switched off, and that will be it; no more using the older Android TV box to ask for today's weather or search for videos.

I think older and low power devices would actually be easier to port: they capture the audio request, send it to the server, receive an audio response from the server and play it. No local "intelligence" at all.

So you could probably replace the whole backend while keeping things compatible as long as you maintain that simple API.