Why not ?

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 ?