> Is file alignment on disk guaranteed, or does it depend on the file system?

I think "guaranteed" is too strong a word given the number of filesystems and flags out there, but "largely" you get aligned I/O.

> The NVMe layer is not the same as the POSIX filesystem, there is no reason we need to throw that as part of knocking the POSIX filesystem off it's privileged position.

I'd say that the POSIX filesystem lives in an ecosystem that makes leveraging NVMe layer characteristics a viable option. More along with the next point.

> Overall you are talking about individual files, but remember what really distinguishes the filesystem is directories. Other database, even relational ones, can have binary blob "leaf data" with the properties you speak about.

I think regardless of how you use a database, your interface is declarative. You always say "update this row" vs "fseek to offset 1048496 and fwrite 128 bytes and then fsync the page cache". Something needs to do the translation from "update this row" to the latter, and that layer will always be closer to hardware.