[fyi80]

Hangouts clients talk HTTP or some network protocol, not Borg/Spanner/GoogleDataCenterMagic

Google doesn't need to build open-source apps, just give users a documented way to send messages on their own accounts to the Talk system.

At most, the extra engineering effort is to implement an authorization and rate-limiting (anti-DOS/spam) system that doesn't depend on obscurity-via-closed-source-client. Most of that probably already exists, for obvious reasons (attackers).

Federation is a different problem, because Google has less control over off-network spammers. Personally, I think it's fine to say that if you want to talk to Google Hangouts users, you need to create a Google account for that purpose. But it would be less evil to have a system by which trusted developers could interoperate, as long as the "trust" filter is non-discriminatory. (Any reasonably public/accountable organization can get an interop key.)

And Google should provide a way for users to send Hangout calls to other servers that choose to implement the Hangout protocol and accept incoming connections from Google, a trusted network. Doing so in its first-party app would be non-evil.