I don't think it was explicitly to compete with Microsoft. Gabe explicitly said when the Windows 8 App Store was announced that Valve was going to ensure Microsoft couldn't lock them out of the desktop market. He said Valve benefitted for PC's openness (up until it was threatened).
Microsoft also had Games for Windows Live at the time, which provided similar functionality to parts of Steam (friends, multiplayer, voice chat, achievements), so with that plus the App Store, one could easily see it as Microsoft coming for their market.
> Mr Newell, who worked for Microsoft for 13 years on Windows, said his company had embraced the open-source software Linux as a "hedging strategy" designed to offset some of the damage Windows 8 was likely to do.
> He said the success of Valve, known for its Half Life, Left4Dead and Portal titles, had been down to the open nature of the PC.
> "We've been a free rider, and we've been able to benefit from everything that went into PCs and the internet," he told the conference. "And we have to continue to figure out how there will be open platforms."
> "There's a strong temptation to close the platform," he said, "because they look at what they can accomplish when they limit the competitors' access to the platform, and they say, 'That's really exciting.'"
If I was going to take on Microsoft I would say a lot of things that were not “I’m going to take on Microsoft,” best not to wake the sleeping giant. You can fix a lot of orgs by attacking them. Also I think Valve is set up as a bit of an anti-Microsoft, a flat(ish?) org structure as opposed to the matrix org structure. Having worked at MSFT I was definitely thinking that these people are going to fumble and a getting into position ready to pick up the ball when that happens might be a good strategy - though clearly a long term one.
I don’t think it’s a coincidence that the story in so many of Valve’s early games—Half-Life 1, Portal, Team Fortress 2—is about how corporate work environments are amusingly terrible.
Microsoft also had Games for Windows Live at the time, which provided similar functionality to parts of Steam (friends, multiplayer, voice chat, achievements), so with that plus the App Store, one could easily see it as Microsoft coming for their market.
> Mr Newell, who worked for Microsoft for 13 years on Windows, said his company had embraced the open-source software Linux as a "hedging strategy" designed to offset some of the damage Windows 8 was likely to do.
> He said the success of Valve, known for its Half Life, Left4Dead and Portal titles, had been down to the open nature of the PC.
> "We've been a free rider, and we've been able to benefit from everything that went into PCs and the internet," he told the conference. "And we have to continue to figure out how there will be open platforms."
> "There's a strong temptation to close the platform," he said, "because they look at what they can accomplish when they limit the competitors' access to the platform, and they say, 'That's really exciting.'"
> https://www.bbc.com/news/technology-18996377