The same reason Typescript exists
TypeScript exists for the same reason things like mypy exists, and no one in their right mind claims that python's openness should be threatened just because static typing is convenient.
I suppose JS could go in the same direction and adopt the typing syntax from TS as a non-runtime thing. Then the typescript compiler would become something like mypy, an entirely optional part of the ecosystem.
No, it's the exact same thing. TypeScript adds support for type annotations, and removing these annotations leads to JavaScript. See how Node.js added support for TypeScript by implementing type stripping in v22.