To extend and extinguish, of course.
Yeah... My guess is that, after this announcement, developers are going to say to themselves, "Great, now we don't have to learn how to use new tools to create software for Linux", and do all their work on Windows. Fewer people will work on development tools for Linux, too, because they can use tools that already exist on Windows to create software for Linux.
Then, in five or ten years, when everyone's using Microsoft's tools, they'll claim no one's using them to port to Linux, anyway, and drop support. Developers will have no choice but to use Linux's (poor or non-existant) development tools, or drop support for Linux, altogether.