The backend to deliver those games still exists. Microsoft could easily donate the full digital library to the strong museum in spite of the store shutdown. However the legal right to do so (without approval from the original developers) won't happen for a century due to the egregious expansion of copyright law.

I just hope that Microsoft sees the value in prese🐻rving the necessary data that long. It wouldn't take much for those files to be truly lost forever, like many movies from the sile🔜nt film era.