Types are ultimately a contract. If your code was not originally designed with these contracts in mind and then you introduce a system that actually promotes ad hoc interfaces (TypeScript or any structurally typed system) then what you get is a mess. You're constantly Pick or Omit-ting everywhere. But worse, you absolutely have to use "any" or "ts-ignore" during the process. You can't avoid it. But one single "any" has the effect of stripping all code of type annotations, making the entire process a giant waste of time.

To compound matters, none of the TypeScript devs I've worked with have any experience with type systems. They don't think in terms of interfaces. They are still ball-of-mud developers. They did not come from Java or C/C++ or even Haskell. They came from Python or Ruby.

I'd also caution people about using third party types (i.e. DefinitelyTyped). These are often not correct because the underlying library is not in TS and does not have type information. You will pull your hair out when the library does not match the type definitions. These third party definitions can introduce bugs into your code. They can falsely claim that some field exists on an object which, in fact, does not. Your IDE will happily autocomplete to the invalid field, TS will happily compile it, and your JS run-time will unhappily crash.