All TypeScript competitors got out-engineered by Microsoft. It’s a triumph of experience over new ideas.
replyHow well designed the initial versions of TypeScript were can be seen by how smoothly later versions were able to build on them, and even after so many major improvements the language has barely a wart (enums probably being the only one).
namespaces are the other one, and that's a fun legacy because Typescript had to implement a half dozen module systems in the early days: no modules (jQuery-era globalThis pollution), AMD, UMD, CommonJS, SystemJS, Typescript's own which was proto-ESM inspired but not ESM, then Typescript's realignment with ESM.
replySomething of its own sign of Microsoft out-engineering some of the hiccups of the ecosystem as a whole. (I started using Typescript < 1 simply because it was the safest and easiest way to write AMD modules also with an eye to UMD or SystemJS output with just a compiler flag change if you needed to ship something compatible outside the house.)