I'm aware of all of the things that you mention, and you can assume that the allocator I am discussing here is reasonably intelligent and does all of those thin

I'm aware of all of the things that you mention, and you can assume that the allocator I am discussing here is reasonably intelligent and does all of those things. Even with everything you mentioned, the point is that the allocator (doing all those things) cannot return free memory to the OS, because the APIs simply don't exist for it: that's what mmap() provides over sbrk(). In the worst case, even for an allocator doing all those things, this can result in an allocator being unable to return any memory: if exactly the last allocation remains alive, sbrk() cannot be called (as you'd free a still-live allocation); but in the case of mmap(), we can free most, or in the best case, all of the memory not in actual use.