1) The actual legal bar for monopoly is a lot higher than you think, otherwise action would be taken against, as examples, Amazon in the digital marketplace, and Waterstones and HMV in the physical. All three companies command a majority share of their markets (online sales, bricks-and-mortar book sales, bricks-and-mortar CD/DVD sales respectively).
2) As soon as Steam comes close to becoming a monopoly, you can bet your bottom dollar EA, Activision/Blizzard, MS etc are going to file legal complaints in the US, UK and Europe. If anyone really thinks their legal teams aren't just waiting for the moment, they're not thinking far enough ahead.