What advantages would you get by being able to reserve memory with a finer than 64-KiB granularity? Especially with 64-bit Windows it seems quite unnecessary to have finer granularity. Am I missing something?
With 64 bits, my real objection is that it’s unnecessarily complicated. There’s some data structure that maps addresses to whatever is mapped at those addresses, and on Windows it also needs to track reserved memory, and it needs to do so at a different granularity. (For huge pages, additional granularities are needed.)