I have seen the alienware alpha go as low as $400, usually for $450. I'm not sure where you are getting your estimates from. It runs pretty much any and every single one of steam's games in medium to high settings though I would recommend an upgrade of ram to 8 gb. I haven't seen how well the Linux variant performs, so it may actually do better because it doesn't Windows weighing it down. You can watch youtube videos on it. Developers don't have to specifically target steam machines because the steam machines simply piggy back onto the steam store. This is also the reason that it doesn't have to be successful market share wise. If the Xbox only sold a few million units, over a 2 year period, that would be disastrous because they rely on market share to attract 3rd parties to port their games over. However Steam machines can pull that off because the companies only need to make a profit off of the machine. The steam machines already have the user base of a 100+ million steam memberships in order to attract 3rd parties to piggy back off of.
As for minimum requirements, there is steam's refund policy if a game doesn't work well. I think Steam also specified that the machines had to be within certain parameters so it would run most games decently. The only ones I haven't seen work on them, are games like AC: Unity or the recent Batman, which were a mess of PC port to begin with.
Steam can also do what google play does, which creates a game is incompatible label on the store page, when it detects your computer hardware or have the game not appear at all.
The points that people bring up as to why steam machine will not be successful, are all valid points. I disagree however because they are all fixable problems. Will it be immediate? No. The reason long term that I believe is potentially very competitive is because of what Valve brings to the table. Steam itself - its massive catalog of games, free online, backwards compatibility going back to the 90s, mod support, steam sales, and the other benefits that pc gamers enjoy.
Again, Steam machines probably its first year out, will start out small and as problems are fixed and more games are compatible, it will pick up in popularity to the point that Microsoft and Sony will have to start shielding their businesses from having their sales siphoned off more noticeably as time goes on. That's my prediction.
The thing is, SteamOS is not Windows, so the developers will have to recode and optimize for it. Why doing it if you only have a few steam machines on the market, and why doing it if some steam machines will run games an others dont. I only see them as a living room expanded thing that receives stream from a big PC and that only some pc gamers will buy. As how it is, they will start slow and continue slow. Anyway, we can talk about it in two years to see who was right.