Steam machines have as target not only consoles but windows as well, this is why valve is promoting steam OS which is free and more and more games are available for it.
As for the upgrades and how the games play, I change card in my system every 3-4 years and I say about mid range cards, same with the console users who change their consoles in that time.
Also if a game release in consoles at the end of their life will have to cut details to be able to play in that console, if a game release in my system's end of life, I just lower the details myself. Now after they stop developing for that console the users are completely cut off new games, while I can still play, even if I have to get details at medium or low (you really need a 10 years old system to turn to low settings but I have seen people do it).
The "convenient console" is a lie, we are no longer in PSX era where I just had to get the disk and power up to play.
Also the inconvenient PC is also a lie now, how hard it is to get a gaming system (even a custom build one), install steam, buy a game, let the steam install it and then play?
As far as I know this is more or less the same thing you do with a modern console, even if you buy a retail game it is most likely you will need to wait for updates to download first.
This is what valve want to do, with big picture, steam machines and controllers. To make it as easy or even easier for a console user to jump ship to the "mother platform" of PC (as a friend says all the time
)