I was confused for a while about where this discussion was going, and what was the broader point. Will try to consolidate thoughts in the interest of making it clearer.
You seem unhappy with POSIX because its guarantees feel incomplete and ad hoc (they are). You like databases because their guarantees are more robust (also true). DBMS over POSIX enables all the guarantees that you like. I'd want to invoke the end-to-end systems argument here and say that this is how systems are supposed to work: POSIX is closer to the hardware, and as a result it is messier. It's the same reason TCP in-order guarantees are layered above the IP layer.
Some of your points re: how the lower layers work seem incorrect, but that doesn't matter in the interest of the big picture. The suggestion (re: microkernels) seems to be that POSIX has a privileged position in the system stack, and that somehow prevents a stronger alternative from existing. I'd say that your gripes with POSIX may be perfectly justified, but nothing prevents a DBMS from owning a complete block device, completely circumventing the filesystem. POSIX is the default, but it is not really privileged by any means.
Is there a DBMS or storage engine intended for a DBMS that does bypass the filesystem altogether? I'm not aware of any, but at the same time I don't have a full grasp of all the storage engines offered.
It almost seems like a ridiculous idea to me for a database component author to want to write there own filesystem instead of improving their DB feature set. I hear the gripes in this thread about filesystems, but they almost sound service level user issues, not deeper technical issues. What I mean by that, is the I/O strategies I've seen from the few open source storage engines i've looked at don't at all seem hindered by the filesystem abstractions that are currently offered. I don't know what a DBMS has to gain from different filesystem abstractions.