As someone who actually works at Microsoft, I can confidently say that a toy third-party language would have absolutely zero chance of getting approved for production code in any major shipping product. The first question that any manager will ask the developer pitching this is, "Where do I find the devs to maintain this codebase long term? And what happens if the language dies upstream?" - and you better have really good answers to those, or else a very convincing story explaining how the productivity boost is worth it.

Even Rust isn't all that easy. That it cleared the hurdle at all on so many teams already (and Windows especially!) is extremely impressive for a piece of tech so new.