Earlier quoted context omitted.
But what about Linux?
Mono! Lots of ASP.Net projects (like Nancyfx or SignalR) are actually built on Mono. I think there's some hope that web dev on Windows isn't dead.
Total cross-platform is a pipe dream. Just like Java or more recently HTML5/JS cross-platform mobile apps, they are almost always a half-baked solution compared to proper native OS apps. The UX and performance will never be consistent enough to make it better than just investing in native development (Note I'm talking about software, not simple content consumption apps where HTML5 in browser is fine).
Having primary operating system -> tablet -> phone is a good enough goal. They should focus 100% and make it as stable ass possible.