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HANDLEs on 64bit windows have 32bits that are valid whereas HANDLES on 32bit windows only have 16bits that are valid

...then use a translation table. If the system is 64-bit, there's certainly enough room and speed for one!

16-bit applications would not be able to handle more than 64k handles either way.

Not as simple as it sounds, HANDLEs are universal on windows. So it requires user mode code to both be in on the charade and to play nice, which it rarely ever does. The issue being things like GetWindowHandle, CreateMutex etc. Passing a Mutex or Event handle to another process is not uncommon on windows. Yes in theory you could just block off the lower 64k Handles as compat handles but it makes code complicated and that leads to unnecessary bugs in code that has to handle it. It's a lot simpler at the end of the day to just ask customers that need that support to just run 32bit windows which has NTVDM support for real mode code and simple 16bit protected mode support that's transparent and just works. So yes they could have done that... but the complication isn't worth it when they have something that works just fine already at no extra cost to development or client; and these apps are being phased out largely anyway. Long story short: the solution sounds simple but once you start plugging all the holes and corner cases it gets insanely complicated very quickly.

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