You see these migrations as companies grow past certain engineer counts if they start out with dynamic languages. Sometimes they might even modify the language itself to bolt on types on it.

Ex: Facebook and adding types with Hack & Flow. Dropbox's work to add types to python. Uber used to be a python and nodejs shop, and now new stuff is done in golang and java. Many nodejs shops go from pure js to typescript or flow.

Gradual typing is useful for companies that don't want to immediately migrate everything at once. If you have the right policy of 'type everything you touch', enforced with a linter, a codebase gets migrated relatively quickly.