Yeah, I know what Typescript is. It's another transpiled language. I don't care what the syntax looks like, the point is you've added a secondary level of knowledge required to operate in a codebase written in it, and most don't fluently know the first. Please tell me more about what I don't know.
Well, if you remove your assumptions about how incapable people are, TypeScript is the way to make large JS code bases maintainable. It doesn’t even change any of the JS syntax, it just adds to it. It’s the opposite of CoffeeScript in many ways.
Typescript is pretty much just annotations and linting. Most of the syntax features have been folded into javascript by now.
In other words it's not a "secondary" level, you only have to learn one level to use it.