If you look at the http://git.savannah.gnu.org/cgit/mediagoblin.git repo, you'll see that Chris hasn't made a release for three years, and that other people are advancing the master branch. Moving from focusing on ActivityPub to focusing on Spritely isn't actually going to change things a lot in that regard.

Spritely may or may not evolve to replace MediaGoblin in some fashion in the future, but the goals of the projects are pretty different, and MediaGoblin has a community to take care of it. In the short term, Spritely is nothing like MediaGoblin.