None of these are distribution channels (with the possible exception of GFWL) so your argument has no merit.
Then why are you bringing up games that use Steam as a DRM and claiming it to be a monopoly for that reason, then disregard all the other games that use a different DRM system that really is on the same coin as Steam just the other side. There are more games that use Securom then there are that use Steamworks. Securom games are tied to your hardware and to activation servers, change your hardware you lose access to you game, servers go away never be able to install your game ever again. Steamwork games are tied to your account, also tied to a server, lose either one lose your games as well. So Securom must be a monopoly, right?
So, what do you want to talk about, Steam as a monopoly as a distribution or steam as a monopoly as a DRM? If you want to talk about Steam as a DRM, then you need to realize that there are far more games that use Securom as a DRM then that use Steam as a DRM. Steamworks isn't used nearly as the only DRM for a game then Securom and Tages are.
As for Steam as a monopoly for Distribution. No one is forced to buy games through Steam, consumers have many many other options to buy their games both physically and through digital distribution. Just how many games are on Steam that you can only buy through Steam? Would it be 25% of the games in the market? no where near that. Now you can't say, well look at this (game), you have to create an account on steam to play the game, cause then you are going back to Steam being used as a DRM, and at that point you have to take into consideration the other forms of DRM and realize they are much bigger then Steam is and are monopoly as well according to your definition.
Just so you know, Steam has 70% of the digital distribution market not because Consumers have no choice, it is because they choose to use Steam as their preference to buy their games. There is nothing that Valve does that forces people to buy their games from Steam. So what is Valve supposed to do? Turn away customers stating they have to many customers?
