But one of our goals was for you to be able to use Svelte's reactivity inside .js/.ts files, since that's one of the things people have been crying out for since we released Svelte 3. For that to work, reactivity has to be opt-in, not opt-out.
And that's how it _should_ be — someone reading the code should be clued into the fact that this `let` won't behave like a normal `let` in JavaScript, and it should be possible to move code between modules (and between modules and components) without worrying about whether a specific file was opted in to certain behaviour.
In other words this...
> This way, the change wouldn't break existing code
...isn't quite right — it would break _all_ your existing code that wasn't in .svelte files.
> If I'm not mistaken, the compiler allows Svelte to define its syntax to anything they want.
On this point specifically: unfortunately not. The code in a .ts file, for example, has to be valid and typecheckable. You can't insert a preprocessing step ahead of the TypeScript compiler. Again though we concluded that this is a good thing, since it means this all works with all existing ecosystem tooling.