I can get the software I find on Steam elsewhere. They aren't exclusive to the platform. And while using Steam's matchmaking service means you're reliant on it staying functional, more often than not the failure is with the game no longer supporting updates to the Steam environment rather than Steam systems simply giving up.

That's not a monopoly, nor is it a problem with Steam as a platform. At least, no more than any networking service has general problems with backwards compatibility and escalating security issues.