Quoting one of your earlier statements:
"Apple's Xcode development tools support building universal binaries. It's a build configuration option in the Xcode UI."
Your earlier assertion was that software developers can smoothly migrate to whatever new microprocessor Apple uses, simply by using Xcode. Do you still assert this?
QT also generates Mac OS X programs. Adobe has used QT in the past, to build Photoshop Album.
Using open-source tools does potentially mitigate risk. Even better is when you can code to open, established standards. Linux is a POSIX-compliant Unix operating system, that is both open-source and adheres to standards. This separates it from other operating systems, that appear to be open but are in fact only open to a certain degree. Lots of software developed for other Unix platforms will compile and run on Linux; this demonstrates the power of standards.
Of course, there are abstractions above the operating system, that are not official standards, so we end up developing to those de-facto standards. Today's de-facto standards can become tomorrow's official standards. So it is also good to avoid dogmatism and pragmatically utilize the best tools for the job. QT may be a candidate for future standardization, as may any new abstractions Apple can develop.