Well, if you write a library in the scriptc subset, it will also work in regular JavaScript runtimes, for free. (Unless you rely on, e.g., catching runtime errors thrown by failed casts, but that's easy enough to avoid.) That seems like a clear advantage over creating a new language.
What I do think would be nice would be if all the tweak-JS-semantics-for-optimization-purposes projects (scriptc, Static Hermes, Nova, possibly Closure Compiler) got together and agreed on a minimal set of rules that TypeScript code should follow in order to be compatible with all of them. Then there could be a push for commonly used npm packages to adhere to those rules, and advertise that they do so in order to be fully optimized.
What I do think would be nice would be if all the tweak-JS-semantics-for-optimization-purposes projects (scriptc, Static Hermes, Nova, possibly Closure Compiler) got together and agreed on a minimal set of rules that TypeScript code should follow in order to be compatible with all of them. Then there could be a push for commonly used npm packages to adhere to those rules, and advertise that they do so in order to be fully optimized.