Another comment in this thread makes a very compelling point: perhaps there is some wisdom to using a language that can be initially dynamically typed, and moved over to static typing later on.

I don't know. I suspect being half-pregnant is a far bigger issue than the technical debt of dynamic typing, whether it stems from being slower to implement new features, spending time choosing the "correct" tech stacks, or spending more time/money hiring qualified engineers.

There's something to be said for the fact that many of the most successful startups in the world are saturated partly or fully with Python and PHP: getting features and new hires off the ground in as little time and money possible is worth its weight in gold.

I've always been very much of the opinion that the smartest way to go is making good use of FFI where it counts, and doing everything else in whatever gets code out the door most effectively. This TypeScript/Hack/MonkeyType trend just seems like a very welcome extension of that.