Look, I'm all for reliability. But, if the manual says: "fsync if you want your data on disk" and we don't fsync, then it is us that are creating the problem.
I think we should come up with a new API that guarantees what people really want. Making the existing API do that on a particular FS is just going to make applications non-portable to any FS that doesn't work that way using existing POSIX API. We've seen this with XFS. Who knows what's lurking out there. Better do the proper thing, fsync and be done with it. Then we can invent the new, better, smarter API.