Disagree. The 'market' is far less important when the good you're selling takes up no tangible shelf space - people have been buying games just fine without Steam for decades before Steam, and still are.

Steam itself, by dint of being another layer between gamers and their games, have to justify its existence and value to them, and generally does through a variety of features as well as its legendary sales. But being a marketplace and distribution network alone does not get you anywhere, not when the cost of setting up shop on the Internet is so low these days.