Moreover OSes designed with higher level languages tend to end up in a very different place even if they start with superficially similar assumptions—compare for example the early Lisp Machine OSes with TOPS-20, or ITS. "Everything's a filesystem" tends to mean "everything's a stream of bytes", which is in some ways a defeat for strong typing at the OS level for all that it can be convenient. I know I've had to write text level parsers for things like the output of multipath -ll (which is not standardized and is intended for human readibility and so changes radically every now and again, but is still the best source of some of that data). Now with Plan 9's FS structure you can arguably turn tree-structured data into a filesystem with files as the leaves, but this doesn't generalize well to non-tree-structured data.
That said I really like the OS, and I'm even willing to admit that Acme is pretty good (just not to my Emacs-tainted taste). But I can't imagine what a relational DB would look like in that idiom, unless it's "SQL query as filename". Individual tables, fine, but joins? Transactions?