Earlier quoted context omitted.
Why this urge to keep up to the latest and greatest? Especially for a transpiler? If it is converting your code just fine, why update?
In the case of Babel - performance. Even with all the fantastic work that has been done on it so far, good luck avoiding massive transpile times for non-trivial ES6 code bases. I imagine TypeScripts (and other) transpilers have the same issue.
An incremental compile takes ~1-2s.