Also, applications written for cross-platform development tools are higher-level and for complying with any new interfaces you may have it's just a matter of rewriting some bindings. In case the change is so radical that it needs architectural changes (like from Carbon to Cocoa), you can always force tool vendors to update their platforms by pulling out Carbon completely (how could they do that if some of their own apps are still on Carbon?)
In fact I would bet that applications on stuff like MonoTouch would've gotten upgraded from Carbon to Cocoa a lot faster than native apps were.
Really, you're setting up a strawman argument.