My completely inexpert musing on this:
Yes, I agree download and transfer will eventually give kfx for those devices capable. As others have suggested, it could be a zip file that's extracted somehow. I think the PW1 is fairly safe from kfx. I imagine the PW1 already would download kfx directly if it could handle it. I don't have a PW1 or any older Kindles myself and would probably stop buying from Amazon if that became the only option to get my hands on non-kfx books. I don't think it will come to that.
I think that kfx coming to Kindle for PC is actually a good thing for getting both the format and its DRM puzzled out. Not only because it's more urgent, but because it's simply easier (for those with the know-how and tools) to see how Kindle for PC handles it.
I'm cautiously optimistic that by the time they force K4PC 1.19 on us and/or start delivering kfx via usb transfer, that the calibre folks as well as Alf & co. will have things sorted.