Something being open source doesn't really have much relation to a technology being open. In this case in particular, it's replacing one open-source API that is easily re-implementable, and has been reimplemented by many parties, by another one that is realistically only available by importing the majority of Chrome into your project.
The point is that NaCL is not browser portable at all. NPAPI is. Therefor, they are depreciating a cross browser plugin architecture for one that is Chrome-only. Which basically means no more plugins at all, unless Chrome captures a ridiculous 50%+ marketshare. Which is actually likely, because Google is doing some real seedy nonsense to get Chrome on every Windows preinstall and bundled with every other freeware program in the universe.
It's a myth that NPAPI is browser portable (at least in the sense that portability is not a massive undertaking). NPAPI doesn't work on mobile platforms and it requires a huge effort to add or even maintain support for a multi-process browser and a modern graphics stack. Even then, the support is a hack that is a constant source of bugs, crashes, and security vulnerabilities. The only thing that's kept it working this long is constant vigilance on the part of browser makers.
