First of all C# doesn't run in the browser without compiling to WASM which has all sorts of caveats.
Second, I don't see how the fact that one could compile TS to C# would suddenly put everyone "on the .NET stack". Does the same risk exist if we can compile JS to C#? Which would work about just as well (i.e. not). And TS is also open source and produces reasonably readable JS, so it's not like there's even any lock-in.
Wouldn't it make more sense to be worried about compiling C# or other languages to WASM? Which I'm also not worried about.