Yes — @humphrey is correct here. Literally being able to add type annotations (and validate that nothing fishy happened in production) took a very short period of time, but everything else was the hard part.

Also, I'd love to hear what a common replacement to Babel looks like. Although we've seen a few faster alternatives, Babel's been working really well since we adopted it, and we haven't had a ton of incentive to try something less battle-tested in the time since.