Meh, when you get to the point where you're exceeding your available address space, there's probably a million other refactorings you had to make to address all of the other architectural changes.
Or, you play by the rules and your old code continues to just work. Might not be optimal, but at least it will run. Your customers will thank you . . . well, we all know they won't, but they won't be speculating about your ancestry in public forums.
I don't remember how many Mac applications were '32-bit clean' and ran without modification when VM was turned on. I do know that some apps had to do significant rework, and that there was a whole generation of abandonware where developers didn't think it was worth the effort and just walked away from their customers.
