Earlier quoted context omitted.
Are you really trying to say people couldn't build complex apps before TypeScript? I know of hundreds of examples that were made before TS... Complex apps still had lots of tests, which basically solve the same problem as TS is trying to address, without having to write a different language than what gets run by the browsers.
How would you compare the overhead of writing tests un JS to the overhead of doing proper TS? Obviously tests are still useful in TS, but not as many I think. Also, the feedback loop of a static type system is, IME, much faster than with tests only.
And my editor does the autocomplete thing seemingly just as well as the autocomplete I get from TS, even when I just write JS code normally.