Most of us already have Steam accounts so I dont think it would hurt your future sales if you sold MW2, Civ5, ETW etc. This way I have to buy them from Steam (I hate buying from retail).
Steam claims to have 25 million accounts, of which 10 million are in use. If you think "most" PC gamers have Steam accounts and that 10 million people is enough people to be "most" then we might as well pack it in and move to consoles now because 10 million or even 25 million is piddly.
In my opinion furure of PC games is bright. As long as there are money to be made, developers will make PC games, also some types of games are almost impossible to play at consoles. Some developers will leave and others will replace them, digital distribution of games might significantly help (IMO) small developers.
As long as there is money to be made is the key phrase. More to the point, as long as the PC provides a competitive ROI.
As a game developer, I can tell you, if it were all just Steam (or all just GFWL or all just Amazon or all just one platform) then I wouldn't make games for the PC anymore.
You assume (wrongly) that "others will replace them". The Amiga, OS/2, Sega Dreamcast and other platforms say differently. Developers will write software for the platforms that provide the best return on their investment.
Steam has these great sales because of competition from Impulse. Where were those great Steam holiday sales in the years before Impulse was doing them? (use archive.org and look at Steampowered.com during the month of December).
Digital distribution is a huge boon to small developers. The PC, however, doesn't have a lock on this. There are iPhone games that have made millions of dollars already. XBox Live Arcade is showing increasing promise. PS3 network is growing.
The PC's primary advantage for developers right now is that they have many different, low cost, avenues to sell their game. By contrast, if I sell my game on the iPhone or Xbox 360, the platform holder is going to take a big chunk.
If I had to sell Elemental on Steam (or Windows market place or Walmart online) exclusively that would be the end because at that point, it's a closed platform and if I'm going to be on a closed platform then I'm going to develop for a closed platform with a better ROI.
I sometimes will read someone comment that I somehow object to Steam and that I'm "whining" about Impulse not having the same marketshare. Obviously, dumb people will say dumb things. I don't object to Steam. I am simply pointing out the logical sequence of events that will occur if the PC effectively becomes a closed platform. It has nothing to do with Valve. Replace Valve with Microsoft or anyone else for that matter and it would be the same thing.