People are drama queens; it would sell just fine as a Steamworks title.
Of course it'll sell; that's not the issue. We as consumers shouldn't even care about that. The question is whenever or not some of us are willing to buy it. I want a functioning game that doesn't rely on 3rd party software (software I despise, I might add) to have it run. I don't want publicity shoved down my throat, I don't want updates to screw everything up and I certainly don't want more applications running in the background to potentially decrease performance. Windows is enough of a cluster !@#$ of a platform as it is.
Steam is a platform I NEVER asked for nor want. If the game comes out on PC, Windows should be its only requirement as far as software goes. Bethesda made Xbox and PS3 versions of their games to expand their audience. What good does having it exclusively on Steam do? I'm not on Xbox, PS3 or Steam... So they're effectively reducing their audience.
Sure, I'll buy the game but it wouldn't make sense for me to buy it if it requires Steam... because I'm not going to play it. It's that simple.
What major titles require you to log on to Steam to play them right now? This isn't a jerk question, I'm honestly not aware of any, but I'm not completely up to speed on the games industry.
Civilization V. There's a CD version, but it installs Steam. Many others have mentioned New Vegas as well.