• Apple has cross-platform apps, e.g. iTunes. Yes, iTunes on Windows is horrible—but only because it is effectively a userland driver stack for old iPods and such, and this prevents them from rewriting it to follow modern architecture. A plain old "app" wouldn't suffer from this problem.
• It's clear from what's been happening to Visual Studio that all IDEs are increasingly expected to be cross-platform. XCode probably would be by now if it wasn't so bound to building Cocoa apps. Handing Apple an editor that's already cross-platform, I don't think they'd drop the capability.