Most of this is on mainboard OEMs. AMD hands them the microcode, what happens next is up to them. They're in the business of selling motherboards. If you've got an old first-gen board that's no longer being sold, good luck - if it's a high end board it might get support. 400 series boards are still on the market, so they'll get support almost universally. In the case of A320, even IF a motherboard manufacturer was interested in adding support, a lot of those boards would still have limitations (some would be 65W and under only, virtually all would only support fairly slow RAM, no overclocking, poor overall performance, etc).
That doesn't even address the BIOS chip size limitations. One thing AMD probably should do is set a minimum standard there when they introduce AM5.