"A day one Xbox one will still be able to run new Xbox one games 10 years from now." - And so will a current steam machine, the problem you're talking about are specifically power-hungry AAA games that push hardware to the limits; if game devs can make a hardware-pushing game for the xbone when it is 10 years old, and can see a good reason in sales to transfer that game onto steam machines, they will make sure to hit minimum hardware specifications to attract as many sales as possible. Steam machines should attract more console game devs, not just PC game devs.
"Plus, there are PLENTY of PC games that just don't translate well to couch-sitting and playing" - they were not designed for steam machines. When the xbone came out did it matter that it didn't have a history of games, of-course not, it's a new machine. It's the same with the steam machine, like the xbone and ps4, the attraction is in the games will be made for it, certainly not for backwards compatibility. Steam just has the advantage of already having a library of games already there.
You're right about the price of these steam machines being underwhelming though. Personally I want to see components that fit into a tight form factor that we consumers can put together, all with quietness and good airflow in mind. What we're seeing now does not seem competitive, and is an obvious indicator of baby steps, no-one taking any risks (probably won't until the idea is more widely understood, which is the hard part)
"Steam machines will not be a PC replacement, but just a way to play certain games in my living room, console style." - This exactly, steam machines are not a PC replacement, to me they have the potential to amalgamate the benefits of pc gaming and consoles in one. I have a steam account, I want cheaper less restricted games, less restricted hardware. I don't want to be in microsofts or sonys eco-systems as they certainly feel restrictive; steam has always felt less restrictive and pc games certainly so.
".. Which is something that has been possible (and really easy to set up) for ages now." - If you're talking about big picture, then yes it shows console gaming as very possible on PC right now. But windows only hampers console gaming, its not an ideal casual environment, and then there is directx, which microsoft control and valve has no influence over. Also the large PC form factor, nobody wants that shit in their living room, if you do have a PC in your living room, then essentially you're trying to make a sleek quiet steam machine that's running windows.
The openness of Linux should allow for a better overall gaming experience from us the user (even if it doesn't right now), and the sooner we change to it, then the easier it is to not be blinded by Microsofts soft cushiony environment. Windows makes for a great pc experience, but Microsoft would rather their xbox be your console experience.
p.s. nice pun ;)