So here's an idea: it's in Microsoft's interest to buy, or clone, Xamarin, and make it work on WP8/Metro as well.
That's because, frankly, platform lock-in is killing Microsoft's chances now that they're the distant third-place touch app ecosystem, for the same reason it helped them as the strong first-place desktop OS. App makers will target the top platform or two, and only those, as long as it's much extra work to go broader. Solid, free-as-in-beer tools to target WP, iOS, and Android all at once make it less work to support WP so more devs will do it.
Having built or acquired the tech to run .NET off Windows, maybe they should release some more of their ecosystem to make it viable for more startups to use .NET. Someone small using some free compilers/libraries and MySQL may eventually be a prospect to buy VS and SQL Server. (And whether they ever sell these startups anything or not, folks releasing .NET libraries make the ecosystem more attractive.)
Who knows; they decided Azure should support Linux and Office should be on every platform, so anything's possible. Times have changed and maybe the argument for closing off ("fully integrating") their dev ecosystem is weaker now.