In 2013, Valve pushed out Steam Machines. I think it's safe to say this was not a success.
With no subsidised hardware as with other consoles, they were expensive. If these were intended to compete in the same market as consoles, they were just poor value for money by comparison. Valve also did not have SteamOS ready by the time the hardware was ready, causing for example manufacturer Alienware to just release theirs with Windows installed, which defeated the purpose of Steam Machines.
The available game library, while great by console launch standards, was way below what your average Windows user was used to from their Steam library. This lead to many who bought the hardware to just dump SteamOS and replace it with Windows. Valve had a hard developer push during their Dev Days event, and some AAA releases did appear, but this did not result in an avalanche of native releases.
So, 2018 and the store pages for the hardware have vanished. Valve admitted that the units were not "exactly flying off the shelves".
Well, let's play a game of wild conjecture!
The prediction is as follows: Valve is preparing to launch a second round of Steam Machines. This time though, they will do the hardware themselves, and the GPU will be AMD.
Things have been happening since the initial launch of Steam Machines. Valve has been pumping money into various technologies in a much more proactive manner than what we're used to what with the generally reactive (or inactive, hur hur) nature of Valve. These efforts have, either by coincidence or deliberately, been chipping away at some of what was missing/wrong with the initial release of Steam Machines.
I doubt Valve has been pouring all this money and work into Linux based gaming just because they love the megabucks from our sweet sweet 1% market share. I want to believe there's more to it. Let's break it down.
Proton, DXVK, Wine: The amount of improvement running Windows games on Linux has seen in the past 2 years is beyond amazing. Thousands of non-native games are suddenly able to run within Steam with only a few clicks, a lot of them delivering over 90% of the performance they do on Windows natively.
The outliers seem to be mostly games that use certain forms of anti-cheat, and there too, Valve have been making efforts, reportedly working with these companies to get the anti-cheat to recognise Proton as a valid platform.
Plus the code remains open source, and can be used outside of Steam. One of the first games to showcase DXVK in action was Overwatch. And at the risk of going off topic, while Epic has been doing its darndest to wrest business away from Valve, these technologies have enabled Linux users to even run the Epic launcher and install Epic Store games.
AMD: Another clue is that Valve has been putting some love into better AMD driver support for Linux. In past years, NVidia was always the better supported brand, but the quality of the open source drivers have improved much in recent years. It's at a point now where it's almost preferable to run your Proton games on AMD hardware.
Also consider the fact that AMD do both GPU and CPU, and can very likely come to a better priced OEM arrangement than split NVidia/Intel arrangements. To me this all adds up to AMD being a solid choice for Valve if they were going to do their own hardware.
Hardware: On the topic of hardware, round 1 of Steam Machines featured all third party hardware, with the exception of the Steam Link and the Steam Controller. Whether this was a plan to be open and Android-like in their business model, or just a sneaky method of offloading risk and testing the waters, we will never know. Valve also recently released their own very successful VR hardware recently, after starting that journey with a third party.
Amusingly, in my original article predicting Steam Machines, one of the breadcrumbs was a patent filing for a modular controller. Some alert folks found another Valve patent, this time for an iteration on the Steam Controller. Still seems to me that Valve is very interested in hardware.
SteamOS: Not much has changed here. Their own Debian based distribution has been receiving updates steadily over the years, even though there's not hardware currently being sold specifically for it.
Other: I mentioned price earlier. Valve Index, despite being (from my standpoint, anyway) quite expensive, has been selling remarkably well. So, maybe price is not as important a point as I am assuming. But either way, if Valve were to ship their own Steam Machine, they can subsidise the hardware, and sell at a more reasonable price, much like the major console manufacturers do.
Missing: The one thing SteamOS/Steam totally lacks is proper support for online media services. You're not going to use your Steam Machine as a full media center unless this is fixed. While not everyone necessarily cares to have all of this from one box, it is still a deal breaker for others.
Unrelated? Bizarrely, Valve is also paying for an overhaul of KDE's compositor. This is either a very loose puzzle piece, or part of a different puzzle. Do they just want games to work better on KDE, or are they planning to improve the SteamOS desktop?
Conclusion: So, add up all of the above and you get an AMD-based Steam Machine manufactured by Valve, running SteamOS, being able to play nearly your entire library of games, and using an updated Steam Controller.