Please re-read mine. How is the number of syscalls (which is the only thing that matters in this context) changing if there's no API change to apps? mmap already exists and already avoids the syscall. DAX "just" makes the implementation faster, but it doesn't appear to have any impact on number of syscalls

As in, if you call read/write instead of using mmap you're still getting a syscall regardless of if DAX is supported or not. Not everything can use mmap. mmap is not a direct replacement for read/write in all scenarios.