> Imagine how many pointers are used in the average program. When we switched over to 64 bits, the memory used by all those pointers instantly doubled.

This is a very real issue (not just on the Windows platform, either) but well-coded software can recover much of that space by using arena allocation and storing indexes instead of general pointers. It would also be nice if we could easily restrict the system allocator to staying within some arbitrary fraction of the program's virtual address space - then we could simply go back to 4-byte general pointers (provided that all library code was updated in due course to support this too) and not even need to mess with arenas.

(We need this anyway to support programs that assume a 48-bit virtual address space on newer systems with 56-bit virtual addresses. Might as well deal with the 32-bit case too.)