It takes a lot of mental and emotional capital in the creative process to produce something that is truly wonderful. Microsoft is systemically incapable of approaching creative perfection. Very few are.
In the case of Typescript the team were working under very difficult constraints - the language must maintain strict compatibility with Javascript, and the output must be executable by a web browser that has never even heard of the language.
It's not just that the output is well-formed and sensible JS, it's that it's structured so similarly to the TS. With a recent EcmaScript version as your output, it's practically just the TypeScript with the type-related keywords & blocks stripped out.
As much as it could provide more and better features by breaking that guarantee, I think it's a brilliant decision & guideline, at least until WebAssembly is far more mature and the space starts to open up.