Nobody said, that this is the ultimate reason :) If I may quote myself: "To draw a conclusion here: Anything is better than nothing. It behaves like pizza, if you eat it when it is cold, it is not as good as a hot one, but pizza is pizza 🍕." :)
There are a lot of good reasons to migrate to TypeScript: type system, interfaces, easier DI, etc.
Just to get the advantage of types, it would be an overkill ;)