The advantage TypeScript has for front-end over something that compiles to JS is that you pretty much know what your code will compile to: with a few exceptions, you just strip out the type annotations. Knowing this, you can accurately predict things like performance.
In contrast, a compiles-to-JS language can produce code that behaves unexpectedly in its non-functional requirements, and in the worst cases crashes where it shouldn't. That will be less of an issue as compilers get better, and WASM should help a lot, but in the current ecosystem TypeScript produces more predictably good results.
That may be true in theory, but I find that people write radically different code in JS and in TS. TS code looks a lot more like C# compared to regular JS code, people use a lot of classes over plain functions and data structures.
TypeScript is not really big and complex, it's just a type system. The only real pain with TypeScript vs. a language with native static types is that TypeScript requires a bit of extra work to fiddle with the compiler, otherwise when it comes to the code it's flexible but can be used in a very simple manner.
Can you say some ways it is more complex than other type systems? I think type systems are inherently complex.
From my perspective, I feel user-facing complexity is quite low in TypeScript in some key ways. You can write your type declarations anywhere, you can import and export them in a familiar JS-module way, you don't need to explicitly type anything in particular so you can be anywhere on the spectrum from very rigorous types to no types at all / `any` everywhere, and finally it has really good error messages so you rarely are stuck with a type mismatch that makes no sense.
