Is EA not a gorilla? The real question is will steam be able to sustain it's business in the long run when bigger fish jump in the water. IMO the only way valve will succeed is if they become a publisher and buy exclusive games.
Valve will likely remain the majority holder of the digital market as long as they continue to appear to be customer focused. The thing that will always hold services like Origin back is that their owners don't answer to gamers, they answer to shareholders that don't know about anything other than profit.