Phoronix article: https://www.phoronix.com/news/Steam-Machines-Frame-2026
Also listed here: https://store.steampowered.com/sale/hardware
Valve has already sent support for the new Steam Controller upstream: https://www.phoronix.com/news/New-Steam-Controller-SDL
It is very hard to make a device that is affordable, compact, efficiently cooled, and modular. Offering complete support is also infinitely more difficult when hardware is not controlled for. GPU and CPU are both customized AMD chips and the power supply is designed for their anticipated power draw.
If you want to swap your GPU, build your own SFF. Hopefully they’ll have SteamOS available for general use soon.
It is also very hard for them to do marketing selling third parties modular parts, it’s much better for them to come up with a product.
If you want to replace parts in your computer so that it doesn’t became useless in 5 years or end up in a landfill when a piece break, build your own SFF