The olde PalmOS had databases as primary storage. Though databases seem to have had capacity for blobs, since apps themselves were stored that way, aside from text files, images and whatnot.
In fact, afaik some mainframe OSes were built around databases.
>WinFS includes a relational database for storage of information, and allows any type of information to be stored in it, provided there is a well defined schema for the type. Individual data items could then be related together by relationships, which are either inferred by the system based on certain attributes or explicitly stated by the user. As the data has a well defined schema, any application can reuse the data; and using the relationships, related data can be effectively organized as well as retrieved. Because the system knows the structure and intent of the information, it can be used to make complex queries that enable advanced searching through the data and aggregating various data items by exploiting the relationships between them.
From what I heard, it was slow, and devs just were not that interested in some clean schema based interface because it complicated their ability to ship; interesting that the modern approach seems to also favor schema on write.
The AS400 operating system had SQL as a key part of the core functionality.
So there is that.
It is just plausible enough to be imaginable. Everything is a table! Write applications using only SQL statements!