Why did Dropbox start anew rather than work with PyPy? Likely because a clean slate not only freed Dropbox of having to assume anyone else's technical debt, but provided a project it could assume complete control over and guide according to its own requirements.
In the short run, it means PyPy now has one less potential source of skilled and valuable input. But in the long run, Python users now have more choice: PyPy with its forward-thinking ecosystem, or Pyston with its emphasis on compatibility.
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