20.2% in July, 18.6% in August, and 17.1% in September...You can blame AMD, though it isn't really AMD's fault, it's the miners keeping prices so heavily inflated. The RX 580, for example, is still a good $80 over launch price. You can blame Vega's issues, but Vega is high end and high end cards are a small percentage of the market, it's the mid range like the RX 580 and GTX 1060 which are the bulk.
People are wondering why developers aren't taking advantage of AMD specific features. What incentive do they have when their market is 3:1 in favor of nVidia? And this is on top of the rather intimate relationship nVidia has with the major game studios.
"People are wondering why developers aren't taking advantage of AMD specific features."
But that is only the PC part of the story is it not? Developers of major titles still tend to develop for consoles first and farm the PC port out to a secondary studio (see Arkham Knight, Mortal Kombat X, Destiny 2, etc.) Consoles currently do run AMD hardware, so one would expect that the studios would optimize for the Xbox/Playstation hardware.
But here is when things run afoul, likely exactly how you suggested in your original post. The secondary studios tasked with giving PC gamers a good experience probably could do a near direct port of the Xbox version to Windows 10 and have a perfectly functional product that could be scaled up in resolution. But NVidia does pay hefty sums to lobby developers into adding NVidia specific "Gameworks" features. Those additions can break functionality on AMD hardware, or at least run less efficiently, ensuring the PC port will run better on NVidia and push PC gamers into purchasing NVidia hardware to get the best frames.
Some have argued that AMDs rise as the choice for miners, would also help it in the PC gaming space. AMDs superiority in mining and "good enough" PC gaming performance would make it the choice of miners who also game. The numbers you present dispel that illusion. In the time frame that Vega 64 has been on the market and also sold out, the RX 580/480s have also been impossible to find. Someone is buying all those AMD GPUs. And yet, when you look at Steam, a significant sampling of the overall PC gamer population, the penetration of AMD GPUs is actually decreasing at an accelerated rate. So all those cards aren't being logged into Steam, and that trend as you say shows no signs of reversing.
I agree with you about some Nvidia Gameworks features causing problems for AMD performance on some titles.
Just looked at DX11 Tom Clancy Ghost Recon Wildlands. Nvidia HBA0+ and GodRays cause significant performance hit on my AMD cards at 4K resolution.
It looks like the AMD GPU share on Steam dropped down to 8.2% in January 2018 around peak of Ethereum price, and started to ramp up again increase in March 2018 as there was a steady decline in Ethereum price from March 2018 to April 2018.
Likely many miners were selling their "mined to death" RX 480 and 580's and Vega 64's in panic, or perhaps they do this deliberately and then buy new GPU's as prices will drop anyhow in prep for next Ethereum price upswing?
Miners seem to still be asking high prices for their used GPU's though.
Ebay prices are NOT dropping here, many more second hand "hardly used" RX 480/580 and Vega 64's now on Ebay used at prices higher than what I could buy them for brand new.
Interestingly the Steam user percentage seems to have leveled off at 15% since start of May 2018 when the Ethereum price turned around and started to rise again.
Hey ajlueke it does indeed look like there is an inverse correlation between Ethereum Price and % of AMD Users on Steam.
Do you give Stock Market Investment advice? .
Thing is though, if miners are selling their cards used to gamers, those gamers may find their cards dying within a few months of purchase, and unfairly blame AMD for that.