I'd be surprised if Matrix is obsolete in a few years. The difference between the Matrix and XMPP spec structure is simply that Matrix is a single curated monolithic spec rather than a set of XEPs. So there's only one true standard at any given point, at a given version (unless someone forks it). We've been evolving it quite rapidly so far, adding modules for the various different use cases out there (http://matrix.org/docs/spec/client_server/r0.1.0.html#featur...), and I don't see that rate of evolution slowing down any time soon.

And even if we did go and entirely rearchitect Matrix (e.g. making it entirely P2P, which is certainly a thought experiment we keep in mind), one would just go bridge it into the existing Matrix ecosystem and migrate as desired.