[stephencanon]

The limiting factor in both Carbon -> Cocoa and 64-bit Photoshop was porting to a completely different API layer, which turns out to be vastly more invasive than changing architectures. Even for very complex userland programs, changing architectures typically doesn't require changes to more than a tiny fraction of the codebase (unless one is porting to something truly exotic, but ARM and x86 are really quite similar from that perspective). You end up spending a few months chasing down bizarre bugs, but the actual scope of the changes is always fairly small.