Interesting, I didn't know that "Source engine" is a Quake engine derivative.
Also, to come up with a highly acclaimed game that would draw people into your new digital store is sure possible, but stunt like this needs some high caliber game like GTA or Doom 3 to succeed.
In other words to start a competing store with Steam might be problematic.
Sure and LOL, they were the first carrying titles like Counter-Strike and Half-life, pretty much forcing people to install Steam in order to play these highly desired games. NOBODY would install Steam without some good game already in. You can try to start a competing company with no such games, good luck.
Most users actually use what is pre-installed, a clear attempt for MS Office to gain new grounds. Why do you think the IE6 is so widespread still ? It is the default bundled browser on Windows XP. This is not a good news for Open Office or Google.