> 1) Performance. We needed something that would consistently work quickly on Instagram's server codebase (currently at several million lines).

That's not really an answer to why you didn't work on mypy, at least to an outsider to the decision making process. Are you saying that you discovered it's just not possible to scale mypy (or at least not without extensive work / more work than building your own solution?)

I can appreciate the choice in context of 2) :)