In fact the "file system mathematically guaranteed to not lose data" is too slow to be used in practice and will implement fsync/fdatasync in the future to regain performance (and in the process it will stop being "mathematicall guaranteed not to lose data").
Clearly the solution is to add fsync/fdatasync calls to every single program so as to negate the performance gains of file system write buffers entirely.
Clearly, the next step after that is for filesystem to start ignoring fsync/fdatasync entirely, because otherwise they would be too slow.