Ok. And why is that important? So one could meticulously add typing to your JavaScript application, and then strip it all out with a regex in order to deploy? What real-life benefit does 'compile by erasure' offer?
That's essentially what all the fast transpilers do -- esbuild, babel etc. They don't typecheck the code at all, they just parse it and strip out the type annotations.
This means that you can run and test code that doesn't typecheck yet -- very useful during development. Also, production builds can run the typechecker in parallel with other build steps. That's how we optimized our build system at work.
But essentially, it's a design choice by the TypeScript team. They've decided that TypeScript shouldn't add runtime features on top of JavaScript, but instead should simply type JavaScript, and I think this has contributed to the success of TypeScript.
I think the point is that TypeScript isn't really "a new language". It's an existing language with an additional feature. The fact that TS maintains the promise that types can be stripped means that TS is upholding a contract that it will not stray from Javascript. It can't, for example, add new language features.
