Dart has an ongoing project (Dart Developer Compiler) which has a goal, among others, to produce readable, idiomatic EcmaScript 6. That is as close to your TypeScript fallback as it can get. (2)
Somebody also demonstrated Dart to LLVM compilation is possible. The language has a decent library for parsing the Dart sources, worst case, if you are that heavily invested in your product, you could also write something that does transpile your codebase. I did try to do it on small scale and specific examples, it is actually not _that_ hard to do, if my business relied on it, it would be certainly within reach.
(1) I'm not sure if you can call it lock-in as it is entirely open source, you can fork it, build it for yourself, change it if you have special needs. The same goes for perl, php, python, go, whatever language you prefer. Yeah, most people don't do it. Why? Because most people don't need it. If you become Facebook-size, it may look better to invest in the PHP toolchain and VM than in transpilers. YMMV.
(2) From the pure technical point of view, I wouldn't call it reassuring that the default fallback platform is JavaScript for so many people (even on the server-side). It is sure depressing that we are stuck with "1" == 1 and wrong ordering of "[1, 2, 10].sort()" for as long as we fall back to JS, and TypeScript does not improve on it.