If you go back and re-listen to some early interviews with Philip Rosedale, he often spoke of how Linden Lab would someday be one of many SL hosting providers, just like there are many WordPress hosting providers. Philip also said that Linden Lab would be the *best* SL hosting provider.
In other words, Linden Lab would become to SL like Automattic is to WordPress.
The SL client code went open source in early 2007. Linden Lab and others began work on the "Open Grid Protocol", which was designed to become an open standard (based on the SL protocol). Everything seemed to be moving along nicely according to plan. And then something happened.
Sometime in 2007 or 2008, Linden Lab changed its plans.
The SL server code never went open source. Work on the Open Grid Protocol quietly died in 2008. Linden Lab CTO Cory Ondrejka was fired in December, 2007; I don't know why, maybe that was just a coincidence.
Philip left Linden Lab and is now working on a new virtual world startup (High Fidelity). It it a bunch of nifty proprietary technical experiments, or is it laying the foundations for a new open metaverse platform? Who knows?
Will Valve build something metaverse-like? It seems like something they might do, and they could too. They already have a game engine and run virtual-goods marketplaces.