Could you explain why you think contributing to PyPy would better achieve the goal of replacing the Python implementation in Dropbox's codebase?

Because it's mature, fast and proven and has good comparability (Achilles hill: C extension support + numpy) , and it would benefit a lot from a high-profile contributor.

I suspect that Dropbox has a bunch of custom C extension code (Pyston puts C extension support as a high priority), which may have been a contributing factor here.