Valve reveals the thought process behind 'Team Fortress'

Speaking in a recent interview, Team Fortress 2 lead designer Robin Walker revealed that Valve was afraid they would go under without an MMO.

The interview with Gamastura, Walker stated that the developer was afraid that without an MMO to call their own, they would not last in the video games industry.
“[When the game shipped],2 Walker explained, “MMOs were the dominant story in the industry, and one concern we had was that we might not be able to survive if we didn’t build one.”

Walker added, “We were starting to feel the same way about micro-transactions as we did initially about MMOs: that our company was at risk if we didn’t have internal experience and hard data on them.”
Team Fortress 2 proved to be an invaluable risk by the company.
“In the end, TF2 has been ended up being one of the most useful tools we’ve ever built to reduce risk in our company’s future,” Walker continued, “The thought that if we hadn’t done it, we’d be here today without any data or experience with service based monetization strategies is quite terrifying.”
It's an interesting take on where the MMO market used to be and where it is now.