I'm not arguing that unions aren't a good choice for Typescript with its given aim to stay close to normal JS semantics. These choices make it difficult to abst

I'm not arguing that unions aren't a good choice for Typescript with its given aim to stay close to normal JS semantics. These choices make it difficult to abstract in raw JS as well (see all the Promises-aren't-monads noise). And yeah, type-level programming is super powerful. You could install it into PS if you wanted, but I don't think that's the goal there.