I don't see how a STEAM box will be a significant win for AMD or Nvidia. Unless Valve plans on making features exclusive to the STEAM box itself, why would I buy one instead of continuing to build/own my own PC? And for people that don't build their own systems, why can Valve offer a better system than established PC builders?
So that leaves Valve indeed offering exclusive STEAM stuff that is not on the PC side of things, doing so risks alienating their existing (and huge) user base no?
Well I think you need to step outside of the shell you are currently in to get the bigger picture. Steam is slave to an operating system that is not designed for gaming and is made by one of it's main competitors. Not a great long-term strategy, especially when that operating system is catering in sales. There are also other competitors potentially looking to get into the living room application based entertainment (Apple) and Gabe correctly sees that the opportunity to thwart / slow Apple is to beat them to the punch with a good, encompassing solution.
The reality, though, is that steambox won't be aimed directly at the people like you, who don't mind dropping $1000 every 2 years on hardware, who don't mind tinkering with settings and/or obscure mods, who want triple-monitor setups, and are comfortable sitting in front of a desk 24/7 to play games. The steambox is initially going after the crowd that hasn't fully embraced PC gaming and/or Steam yet, who likes the idea of gaming on a PC but is turned off by the initial high costs for a good setup and simply wants to turn on the machine and play games without having to worry about updating drivers or tweaking graphics options.
Steambox is irrelevant, people who play steam games already have a PC for that. Unless you think Steambox is capable of eating into the real console crowd, and beat the likes of xbone and ps4..
Eat into their sales somewhat maybe, but like I said about more likely coexist along side consoles and expand it's own user base (as well as time spent by users). But being irrelevant is incorrect, long term it is very relevant. People like me will not spend the rest of our lives building oversized desktops to play games. I'm nearly done with dropping hundreds of dollars every year on upgrades. Steam provides most of which I like in PC gaming - workshop for mods, organization of games, easy install, keyboard & mouse, better graphics and performance than consoles on capable machines. If a steambox can provide all that, awesome I'm done sinking hundreds of dollars into upgrading every new GPU cycle. My priorities, just like many other people who game, are changing and I value different experiences more now than I did 3-4 years ago.
I was under the impression that "steam box" was an open specification and Valve would let anyone build such a machine.
I think Valve is going to go the route of Google (Nexus) and have their own hardware in addition to third party hardware.