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.