And it wasn't exclusive, they sold other people's games on there too. Then they helped kickstart the indie gaming explosion with the Potato Sack. Then they started trying to make Linux a better gaming platform to provide an alternative to being locked in to MS. Then they released a non-exclusive VR platform against a competitor that was exclusive.
Say what you want about Valve having a monopoly and that being bad for the market, because that's generally true, but my opinion is that Valve has demonstrated a sincere interest in doing good things for PC gaming, not just what's good for Valve.