Interesting question, but doesn't a "metaverse" require interoperable applications to some extent?

I think the Zuck use case was buying a t-shirt for a virtual avatar and I want to use that t-shirt in other applications. There is a matter of interoperability required at the level of the data that sits behind both applications as it pertains to the user that sits on the other side of those applications. (meaning just sharing data isn't enough, there is user level context as well).

And if you reduce interoperability to a single entity owned platform (like Steam), well other platforms will exist and the interoperability problem will be recreated.