The number of truly uninformed people in this thread astounds me...
I'm going to establish two things now:
1. A monopoly means that a business/person/monkey has exclusive rights to the sale/distribution of games/resources/bananas. By definition, Steam can not have a monopoly over the digital distribution market. It can hold the majority, but it can not have a monopoly until all other digital distributors disappear.
2. EA is not really planning on "competing" with Steam. In order to compete with Steam, they would need to offer the same content for a competitive price, and they know that they would never win against Steam in a head-on battle. So they're going to low-ball their way into the game. Or have you not realized how many things are going to be exclusive to Origin? Deluxe Edition of Mass Effect 3? Only available on Origin. And it costs $10 more for 3 pieces of cosmetic gear. And I think the only reason that Mass Effect 3 is still rumored to be on Steam at all is because someone at EA realized that not releasing it on Steam would take away a lot of their sales on release.
Everything comes down to money. If Origin stays afloat, and it probably will because of how much money EA is willing to sink into stupid projects, then it will get more and more EA exclusives as time goes on and they will nickle-and-dime every last customer and get away with it because they will be the only provider of the game. And that, my good fellows, is a monopoly.
Also, if Origin actually does start to make a return for EA, then it might cause other publishers to start up their own exclusive Digital Distribution consoles as well which essentially means two things - Steam will shrink more and more, and the customers are going to need to remember more and more username + password combinations for more and more random accounts.