I think it's heavily implied ("its real contribution is a set of interfaces, protocols, and components from which an information management system can be built"

I think it's heavily implied ("its real contribution is a set of interfaces, protocols, and components from which an information management system can be built") that given enough buy-in, existing content stores could hook into the Upspin namespace, but that's of course a strategic decision that such content stores must evaluate. Aside from the content-addressed bit, it sounds to me like in many ways this idea is similar to OAuth or even SAML both in purpose and ambition, prescribing a standard way one can punch a delegated-access hole into restricted space. Upspin would then act like an overlay network to locate files, then hand the authorization decision down to the implementing system. Back in the day, lots of people bought into OAuth because not doing so offered no competitive advantage, but rather resulted in an explosion of tools and integrations that greatly benefited all those services. This believes that others would be tempted as well.