They have changed the video game market completely. They made games more accessible to people, made it so independent developers can reach a market. The question is, however, as a company how is valve. Because valve basically abandoned their entire fan base and stopped developing the series that people want to continue or at least finish. That's where I think they failed.
Valve Corporation (formerly Valve Software, commonly referred to as Valve) is an American video game development and digital distribution company headquartered in Bellevue, Washington, United States. Its European-based office opened in 2012 in Luxembourg. Founded in 1996 by former Microsoft employees Gabe Newell and Mike Harrington, Valve developed critically acclaimed video game series Half-Life, Portal, and Left 4 Dead. It has also developed and maintains the Source engine for which most of its games run on, and the software distribution platform Steam, which has led to the Steam Machine, a line of pre-built gaming computers running SteamOS. More: en.wikipedia.org.