Here's plenty of rivals, but steam got in first and carved out a huge chunk of market-share for themselves. Things like steam chat, gifting, trade, forums, etc connect their users, so that they don't want to switch to other services. They also offer a lot of services to devs, such as the obvious file hosting and (very fast, global) distribution, patching, public/private betas, branches, micropayment API, VOIP APIs, NAT punch-through with free proxy fallbacks, matchmaking, lobbies, etc... On top of that, they've (seemingly?) got the majority of digital PC consumers on the hook, making their store a place you really want to be available from.
All this makes for a very strong feedback loop, keeping it the dominant force.

EA, Ubisoft and the smaller 'indie' versions don't even have feature parity... Along with having less consumers visiting them. Some of the indie ones are much more user friendly, such as having no DRM, or letting you use them without an account, but are competing as niche operators, rather than actual mainstream competition.

The Witcher 2 numbers on GoG might be a bit inflated, given that CDProjeckt made The Witcher and GoG, so its a favorite among The Witcher players. I wouldn't expect those numbers with an unrelated game developer.

For one thing, are there even enough PC games coming out to support very many major portals? This is a case where the "pie" is a fixed size, so jumping into the mix means you'll be fighting for a small slice of it in the best case scenario. I can't see a new portal bringing in new gamers that aren't buying games now. GoG may have done that to a point with no DRM, but what's left to add?

My guess for the biggest 'alternative' to Steam would be Big Fish Games. They focus on a specific type of game - casual, lasting 4-6 hours, that they can sell cheaply and profit: Hidden Object, Match-3, Time Management, Road Builders, etc... I would not be surprised at all to find out that HOPA's are currently the most profitable segment of PC gaming right now, though production costs from the major developers are rising: cinematic cutscenes, fully voiced, etc... The days when you could slap some clip art on a pile are pretty much done. Even Match-3 games have storylines with voiced characters now.

Possible, but the numbers of the independent Defender's Quest were similar.

Allthough a game like defender's quest is probably more likely to get a favorable reception with the gog.com users (who enjoy retro/classic games) than it would with the more diverse Steam userbase.