Here's my guess as to what happens:
1) I feel like a big 2-4 will develop. Quite simply, developers/publishers won't want to put their game on all sorts of different platforms. It's a hassle. Also, those sales we see now probably aren't something they want. It's ok for Valve to do with their games since they're the developer, but 3rd-parties don't like their games selling for $5. We're already seeing people saying they'll wait for the 75% off sale. I believe this will lead to the following consumer market:
- Lots of first day sales for the must have games
- People then wait for 75% discounts for other stuff
Stardock's business model with the pre-order betas might help tremendously here as a business strategy.
2) I feel that if Stardock or Gamersgate gets their foot in the door fully, there will be a backlash against Steam among some consumers. Not all but some. This may take a short-term loss and some aggressive sales. Steam's DRM is annoying, especially when compared to Impulse/Gamersgate DRM.
3) I think Gamersgate is the most likely to cave. Reason: Valve and Stardock are bigger ,and Valve has its development success, and Stardock has its other business. Both companies can absorb a short-term loss more then Paradox, who I think is much smaller. Gamersgate would survive but would give up on being a major 3rd-party site.
4) It is possible for EA or Activision to step into the game still, Activision more likely, if they wanted to. I don't know if they'd really want to though.
My end prediction: in 3-5 years, we see a two horse, maybe three horse race develop, with Valve having about 70% of the market share, Impulse about 25%, and 5% on other places. If we get a clear answer to the DRM debate in that time as well- if DRM wins, Valve will have a monopoly, if DRM loses, Impulse will either have a majority, or Valve will drop it's DRM.
We will see the PC equivalent of the console wars develop.