> if Windows ABI is so stable that 20-year-old programs are guaranteed to run That's not actually true; there are no guarantees. Microsoft does a best effort

> if Windows ABI is so stable that 20-year-old programs are guaranteed to run That's not actually true; there are no guarantees. Microsoft does a best effort to ensure the majority of applications continue to work. But there are billions of applications, they're not all going to work. Many applications don't even adhere to the Win32 API properly. Microsoft will sometimes, if the app is important enough, ensure even misbehaving applications work properly.