On the flip side, if Apple wants to take the consoles head-on (a fairly safe assumption), they're going to need strong console-level games. Partnering with Valve puts them on mostly equal footing with Microsoft and Sony in terms of publisher relationships, as well as a few headliner exclusives.
They have a nice user base and if they'll adapt to WinStore coming out with proper counter-measures and marketing approaches (and maybe some new ideas), I think there's nothing they should be afraid of.
Plus, considering Microsoft's history with PC gaming it would take a great deal for me to switch to their platform from Steam. Game for Windows Live anyone?
Unless Microsoft ups their game (bad pun!) in terms of actually supporting PC gaming then I would think Valve will be just fine. I'm willing to bet Xbox Live integration on Windows 8 is not a selling point to Valve's market.
If someone from Microsoft suggested I should switch from Steam to their platform I would laugh in his face.
How was Dropbox able to "survive" when Apple has their cloud (and I'm sure Windows will feature something similar as well)? If I remember correctly, Apple wanted to buy them and they said no.
The bottom line, it can be done. Others have proven it.
Windows Store, by design, removes the ability for Steam to function. It's technically possible to run Steam on Win8-x86, but constrained to the "legacy" desktop and denied integration of any kind with Metro. They'll be shut out completely with ARM. Their momentum will carry them for a while, but provided Metro isn't a massive failure Steam's current business model will be unviable within 5 years of Win8's launch. If they are to survive, Valve /must/ get off the PC.
Or more seriously, if Valve wanted a hardware partner then why wouldn't they work with Apple?
I hope Apple has learned from that lesson, but on the other hand, Apple is an environment focused on operating systems and hardware, and not necessarily run by the right people to be deciding which games to green light.
Don't get me wrong, I'm sure it still sells after two years, but the whole "brand new game that takes the market with storm" phrase has long since passed.