I don't see the audience for a low consumption, but garbage at performance GPU. I mean everyone I know, everyone I've met, every single gamer wants faster performance, over pretty much anything. Other things are a consideration, but fall way by the side when compared to performance.
Now in low end systems designed for video consumption and maybe very light gaming, having low consumption card at a cheap price is important, but we are talking about a specific niche market and looking at prices up to $100 at most.
If AMD did do a really consumption friendly card, lets call it RX 580 ECO operating at say max boost of 1100, default clock of 1000, same costs, same design, the only difference the clock and consumption, I bet you less than 0.5% will buy the ECO version over the much faster normal 580.
And again ultimately you can downclock and downvolt your 580 yourself and have it be more efficient, but significantly slower.