I might even go as far as putting on a "tin foil hat" and state that Typescript is a trick to lock you into their proprietary ecosystem of expensive tools that everyone have to buy into just to communicate with you once you are locked in.
I only see advantages for static typing for a small cost of time: you prevent most of Type Error from happening, you get automatic auto-completion, refactoring errors are easier to catch and feel safer. Either way, you need to specify the type of the arguments in the documentation of your function (I can sometimes even find the C# LINQ's function I'm looking for just by looking at the return type of the list of function).
You could even use Flow and Typescript to catch
Aside from the proprietary reasons (which I believe are unfounded since the whole Typescript ecosystem is opensource), why do you think static typing doesn't belong in Javascript ?
https://github.com/Microsoft/TypeScript is uh... right there. Check the license. Visual Studio code runs Textmate snippets and has a very sublime-like extension system.
There is no vendor lock in.
What do you think is the advantage of such a thing? IME they tend to end up as an ad hoc, informally-specified, bug-ridden, slow implementation of half of a type system.